Extreme ultraviolet light source

ABSTRACT

The present invention provides a reliable, high-repetition rate, production line compatible high energy photon source. A very hot plasma containing an active material is produced in vacuum chamber. The active material is an atomic element having an emission line within a desired extreme ultraviolet (EUV) range. A pulse power source comprising a charging capacitor and a magnetic compression circuit comprising a pulse transformer, provides electrical pulses having sufficient energy and electrical potential sufficient to produce the EUV light at an intermediate focus at rates in excess of 5 Watts. In preferred embodiments designed by Applicants in-band, EUV light energy at the intermediate focus is 45 Watts extendable to 105.8 Watts.

[0001] This application is a continuation-in-part of U.S. Ser.No.10/384,967 filed Mar. 8, 2003, Ser. No. 10/189,824 filed Jul. 3,2002, U.S. Ser. No. 10/120,655 filed Apr. 10, 2002, U.S. Ser. No.09/875,719 filed Jun. 6, 2001 and U.S. Ser. No. 09/875,721 filed Jun. 6,2001, U.S. Ser. No. 09/690,084 filed Oct. 16, 2000; and claims thebenefit of patent application Ser. No. 60/422,808 filed Oct. 31, 2002and patent application Ser. No. 60/419,805 filed Oct. 18, 2002; all ofwhich is incorporated by reference herein. This invention relates tohigh-energy photon sources and in particular highly reliable x-ray andhigh-energy ultraviolet sources.

BACKGROUND OF THE INVENTION

[0002] The semiconductor industry continues to develop lithographictechnologies, which can print ever-smaller integrated circuitdimensions. These systems must have high reliability, cost effectivethroughput, and reasonable process latitude. The integrated circuitfabrication industry has recently changed over from mercury G-line (436nm) and I-line (365 nm) exposure sources to 248 nm and 193 nm excimerlaser sources. This transition was precipitated by the need for higherlithographic resolution with minimum loss in depth-of-focus.

[0003] The demands of the integrated circuit industry will soon exceedthe resolution capabilities of 193 nm exposure sources, thus creating aneed for a reliable exposure source at a wavelength significantlyshorter than 193 nm. An excimer line exists at 157 nm, but opticalmaterials with sufficient transmission at this wavelength andsufficiently high optical quality are difficult to obtain. Therefore,all-reflective imaging systems may be required. An all reflectiveoptical system requires a smaller numerical aperture (NA) than thetransmissive systems. The loss in resolution caused by the smaller NAcan only be made up by reducing the wavelength by a large factor. Thus,a light source in the range of 10 to 20 nm is required if the resolutionof optical lithography is to be improved beyond that achieved with 193nm or 157 nm. Optical components for light at wavelengths below 157 nmare very limited. However, effective incidents reflectors are availableand good reflectors multi-layer at near normal angles of incidence canbe made for light in the wavelength range of between about 10 and 14 nm.(Light in this wavelength range is within a spectral range known asextreme ultraviolet light and some would light in this range, softx-rays.) For these reasons there is a need for a good reliable lightsource at wavelengths in this range such as of about 13.5 nm.

[0004] The present state of the art in high energy ultraviolet and x-raysources utilizes plasmas produced by bombarding various target materialswith laser beams, electrons or other particles. Solid targets have beenused, but the debris created by ablation of the solid target hasdetrimental effects on various components of a system intended forproduction line operation. A proposed solution to the debris problem isto use a frozen liquid or liquidfied or frozen gas target so that thedebris will not plate out onto the optical equipment. However, none ofthese systems have so far proven to be practical for production lineoperation.

[0005] It has been well known for many years that x-rays and high energyultraviolet radiation could be produced in a plasma pinch operation. Ina plasma pinch an electric current is passed through a plasma in one ofseveral possible configuration such that the magnetic field created bythe flowing electric current accelerates the electrons and ions in theplasma into a tiny volume with sufficient energy to cause substantialstripping of outer electrons from the ions and a consequent productionof x-rays and high energy ultraviolet radiation. Various prior arttechniques for generation of high energy radiation from focusing orpinching plasmas are described in the background section of U.S. Pat.No. 6,452,199.

[0006] Typical prior art plasma focus devices can generate large amountsof radiation suitable for proximity x-ray lithography, but are limitedin repetition rate due to large per pulse electrical energyrequirements, and short lived internal components. The stored electricalenergy requirements for these systems range from 1 kJ to 100 kJ. Therepetition rates typically did not exceed a few pulses per second.

[0007] What is needed are production line reliable, systems forproducing collecting and directing high energy ultraviolet x-radiationwithin desired wavelength ranges which can operate reliably at highrepetition rates and avoid prior art problems associated with debrisformation.

SUMMARY OF THE INVENTION

[0008] The present invention provides a reliable, high-repetition rate,production line compatible high energy photon source. A very hot plasmacontaining an active material is produced in vacuum chamber. The activematerial is an atomic element having an emission line within a desiredextreme ultraviolet (EUV) wavelength range. A pulse power source,comprising a charging capacitor and a magnetic compression circuitcomprising a pulse transformer, provides electrical pulses havingsufficient energy and electrical potential sufficient to produce the EUVlight at an intermediate focus at rates in excess of 5 Watts on acontinuous basis and in excess of 20 Watts on a burst basis. Inpreferred embodiments designed by Applicants in-band, EUV light energyat the intermediate focus is 45 Watts extendable to 105.8 Watts.

[0009] In preferred embodiments the high energy photon source is a denseplasma focus device with co-axial electrodes. the electrodes areconfigured co-axially. The central electrode is preferably hollow and anactive gas is introduced out of the hollow electrode. This permits anoptimization of the spectral line source and a separate optimization ofa buffer gas. In preferred embodiments the central electrode is pulsedwith a high negative electrical pulse so that the central electrodefunctions as a hollow cathode. Preferred embodiments presentoptimization of capacitance values, anode length and shape and preferredactive gas delivery systems are disclosed. Special techniques aredescribed for cooling the central electrode. In one example, water iscirculated through the walls of the hollow electrode. In anotherexample, a heat pipe cooling system is described for cooling the centralelectrode.

[0010] An external reflection radiation collector-director collectsradiation produced in the plasma pinch and directs the radiation in adesired direction. Good choices for the reflector material aremolybdenum, palladium, ruthenium, rhodium, gold or tungsten. Inpreferred embodiments the active material may be xenon, lithium vapor,tin vapor and the buffer gas is helium and the radiation-collector ismade of or coated with a material possessing high grazing incidencereflectivity. Other potential active materials are described.

[0011] In preferred embodiments the buffer gas is helium or argon.Lithium vapor may be produced by vaporization of solid or liquid lithiumlocated in a hole along the axis of the central electrode of a coaxialelectrode configuration. Lithium may also be provided in solutions sincealkali metals dissolve in amines. A lithium solution in ammonia (NH₃) isa good candidate. Lithium may also be provided by a sputtering processin which pre-ionization discharges serves the double purpose ofproviding lithium vapor and also pre-ionization. In preferredembodiments, debris is collected on a conical nested debris collectorhaving surfaces aligned with light rays extending out from the pinchsite and directed toward the radiation collector-director. Thereflection radiation collector-director and the conical nested debriscollector could be fabricated together as one part or they could beseparate parts aligned with each other and the pinch site.

[0012] This prototype devices actually built and test by Applicantsconvert electrical pulses (either positive or negative) of about 10 J ofstored electrical energy per pulse into approximately 50 mJ of in-band13.5 nm radiation emitted into 2π steradians. Thus, these tests havedemonstrated a conversion efficiency of about 0.5%, Applicants estimatethat they can collect about 20 percent of the 50 mJ 13.5 nm radiation sothat this demonstrated collected energy per pulse will be in about of 10mJ. Applicants have demonstrated 1000 Hz continuous operation and 4000Hz short burst operation. Thus, 10 Watt continuous and 40 Watt burstoutputs have been demonstrated. Using collection techniques designed byApplicants about half of this energy can be delivered to an intermediatefocus distant from the plasma source. Thus providing at least 5 Watts ofin band EUV light at the intermediate focus on a continuous basis and atleast 20 Watts on a burst basis. Applicants have also shown that thetechniques described herein can be applied to provide outputs in therange of 60 Watts at repetition rates of 5,000 Hz or greater. At 2000Hz, the measured pulse-to-pulse energy stability, (standard deviation)was about 9.4% and no drop out pulses were observed. The electricalcircuit and operation of this prototype DPF device is presented alongwith a description of several preferred modifications intended toimprove stability, efficiency and performance.

[0013] In other embodiments the plasma may be produced in other plasmapinch devices such as a conventional z pinch device, a hollow cathodez-pinch or a capillary discharge or the plasma may be produced with apulsed gas discharge laser beam. The pulse power or each of thesesources is produced with a pulse power system as described herein and ineach the EUV light preferably is produced collected and is preferablydelivered to an intermediate focus using one or more of the techniquesdescribed herein.

[0014] The present invention provides a practical implementation of EUVlithography in a reliable, high brightness EUV light source withemission characteristics well matched to the reflection band of theMo/Si or Mo/Be mirror systems. Tests by Applicants have demonstrated animproved electrode configuration in which the central electrodeconfiguration in which the central electrode is hollow and configured asa cathode. For this configuration the hollow cathode produces its ownpre-ionization so special pre-ionization is not needed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is an electrical drawing of a pulse power system useful asa power source for EUV and soft x-ray sources.

[0016]FIG. 1A shows structure elements of a pulse transformer.

[0017]FIGS. 1B and 1C show test data.

[0018]FIG. 1D shows a reverse polarity pulse power source.

[0019]FIG. 2A shows electrical features of a dense plasma focus EUVdevice.

[0020] FIGS. 2A(1) and 2A(2) are cross-section drawings of a plasmapinch prototype EUV device.

[0021]FIG. 2A(3) shows the prototype with vacuum chamber.

[0022]FIG. 2A(4) shows flow cooled equipment.

[0023]FIG. 2A(5) shows effects of flow on output.

[0024] FIGS. 2A(6)-(20) show performance data.

[0025]FIG. 2A(21) shows a special DPF configuration.

[0026]FIG. 2B shows features of a conventional Z-pinch device.

[0027]FIG. 2C shows features of a hollow cathode Z-pinch device.

[0028]FIG. 2D shows features of a capillary discharge device.

[0029]FIGS. 3A and B show xenon spectra.

[0030]FIGS. 4, 4A and 4B show features of a laser produced plasmasystems.

[0031]FIG. 4C shows a hybrid EUV system.

[0032] FIGS. 5A-C shows methods of making a debris collector

[0033]FIGS. 6A and 6B show features of a second debris collector.

[0034]FIGS. 7A, B, and C shows features of a third debris collector.

[0035]FIG. 7A is a prospective drawing of a hyprobolic collector.

[0036]FIG. 7B shows a portion of the EUV beam produced by an ellipsoidalcollector.

[0037]FIG. 7C shows a portion of the EUV beam produced by a hyproboliccollector.

[0038]FIG. 8 shows the 13.5 nm lithium peak relative to reflectivity ofMoSi coatings.

[0039]FIGS. 9A, B, and C show a nested conical debris collector.

[0040]FIG. 10 shows a xenon spectra a multi-layer mirror spectra.

[0041]FIG. 11 is a chart showing reflectivity of various materials for13.5 mn ultraviolet radiation.

[0042]FIGS. 11A, B, C, D, and E show collector designs.

[0043]FIG. 12 is a drawing showing a technique for introducing sourcegas and working gas.

[0044]FIG. 13 is a time chart showing the anode voltage and EUVintensity.

[0045]FIGS. 14A, 14B, 14C and 14D show the effect of various centralelectrode designs on the plasma pinch.

[0046]FIG. 15 is a drawing showing a technique for using RF energy tooperate lithium vapor source gas.

[0047]FIG. 16 shows a heat pipe cooling technique for the anode in apreferred DPF device.

[0048]FIG. 17 shows gas control techniques.

[0049]FIGS. 18A, B, C, and D show techniques for controlling active gasand buffer gas in the vacuum vessel of preferred embodiments.

[0050]FIG. 19 shows a tandem ellipsoidal mirror arrangement.

[0051]FIGS. 19A, B, and C show the shape of the EUV profile at justdownstream of two focuses.

[0052]FIGS. 20, 20A, 21 and 22 show a technique for water-cooling of theelectrodes.

[0053]FIGS. 23, 24, 25 and 26A show techniques for reducing electrodeerosion.

[0054]FIGS. 27A and 27B show a maintenance technique.

[0055]FIGS. 28A and B show the use of magnets to control the pinch.

[0056]FIGS. 29A, 29B, and 30

[0057]FIG. 31 shows a preionization technique.

[0058]FIG. 32 shows the effects of preionization turning.

[0059]FIG. 33 shows advantages of dense plasma focus.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Hot Plasmas

[0060] To produce light in the spectral range of 13-14 nm from plasmarequires a very hot plasma corresponding to temperatures of in the rangeof several thousand degrees Celsius. Plasmas at these temperatures canbe created by focusing a very high power (very short pulse) laser beamor a high energy electron beam on the surface of a metal target. It isalso possible to produce very hot plasma in a gas with electricdischarges using any of several special discharge techniques which focusor pinch the plasma. These techniques included (1) a dense plasma focustechnique (2) a regular Z-pinch technique, (3) a hollow cathode Z-pinchand (4) a capillary discharge technique. All of these techniques arediscussed in greater detail below. For use as a lithography light sourcefor integrated circuit fabrication the light source and the power supplyfor it should be capable of continuous, reliable, round-the-clockoperation for many billions of pulses. This is because the lithographymachines and the associated fabrication lines are extremely expensiveand any unscheduled down time could represent losses of hundreds ofthousands of dollars per hour.

Atomic Sources for 12-14 nm EUV Spectral Lines

[0061] As stated in the background section of this specification goodmirrors are available providing reflectances in the range of about 70%or higher in the wavelength the range of between about 10 and 14 nm.These mirrors typically provide reflectances at these high values onlyover a more narrow spectral range within the 12 to 14 nm range. Forexample, the mirror depicted in FIG. 11A provides reflectances of about70% over the spectral range of about 13.2 to 13.8 nm. This mirror can bedescribed as having a reflectance of about 0.7 at 13.5 nm with a FWHMbandwidth of 0.5 nm. These mirrors can be effectively utilized forlithography machines for future integrated circuit lithography. Plasmaproducing devices described below, including those shown in FIGS. 2Athrough 2D produce spot plasmas with extremely high temperatures in therange of several thousand degrees Celsius but the spectrum of lightemitted is spread over a very wide range. To produce light within thedesired range of about 13-14 nm, hot spot plasma should include anatomic target material with spectral lines in the 13-14 nm range.Several potential target materials are known including xenon, lithiumand tin. The best choice of target materials involve trade-offs relatingto spectra available, efficiency of conversion of plasma energy toenergy in the desired spectrum, difficulty of injecting the target intothe plasma region, debris problems. Some preferred target and techniquesfor dealing with these issues are discussed below. (The reader shouldunderstand that all elements produce spectral lines at high temperatureand that these lines are well documented so that if light at otherwavelengths is desired, it is a rather straight forward matter to searchthe literature for a suitable target material which when heated in aplasma will produce a good line at the wavelength of interest).

Xenon

[0062] Xenon is a preferred atomic target. It is a nobel gas therefore,it does not present a debris problem. It has relatively good spectrallines within the 13 to 14 nm range as shown in FIGS. 3A and 3B. FIG. 11Ashows EUV Xe spectra measured by Applicants.

[0063]FIG. 3A shows a measured single pulse spectrum. FIG. 3B shows acalculated theoretical Xe spectron. It can can be added as a constituentpart of the buffer gas in the discharge chamber, or it an be injectedclose to the discharge or pinch region so that its concentration isgreater there. It can also be cooled to below its boiling point andinjected into the discharge or pinch region as a liquid or a solid sothat its atomic concentration is greatly increased in the plasma.Certain xenon compounds (such as xenon oxifluoride) might also make goodtarget materials.

Lithium

[0064] Lithium is also well known as a potential target material. It isa solid at standard temperatures and it does pose a debris problem.Also, special techniques must be devised when adding atomic lithium tothe discharge or pinch region. Some of those techniques are described inthe parent patent applications and in prior art sources and othertechniques are described below. Lithium can be injected into the chamberas a solid, liquid or a vapor.

Tin

[0065] Tin is also a preferred target material since it has some intensespectral lines in the desired range. However, like lithium it is a solidat standard temperatures and does pose a debris problem since it couldpotentially plate out on optical surfaces.

Pulse Power System Electrical Circuit Need For Long Life Reliable PulsePower

[0066] Several prior art pulse power supply systems are known forsupplying short electrical high voltage pulses to create the dischargesin these devices. However, none of these prior art power suppliesprovides the reliability and control features needed for high repetitionrate, high power long-life and reliability needed for integrated circuitlithographic production. Applicants have, however, built and tested apulse power system relying in part on technology developed by Applicantsfor their excimer laser light sources. These excimer lasers producing248 nm and 193 nm light, are currently extensively used as light sourcesfor integrated circuit fabrication. A long life reliable pulse powersystem for EUV devices built and tested by Applicants as part of afourth generation plasma focus device is described in the followingsections.

[0067] A description of the electrical circuit diagram of this preferredpulse power system with reference to FIG. 1 and occasionally to FIGS.1A, 2A and B is set forth below. A conventional approximately 700 V dcpower supply 400 is used to convert AC electrical power from utility 208Volt, 3 phase power into approximately 700 V dc 50 amp power. This powersupply 400 provides power for resonant charger unit 402. Power supplyunit 400 charges up a large 1550 μF capacitor bank, C-1. Upon commandfrom an external trigger signal, the resonant charger initiates acharging cycle by closing the command-charging switch, S1. Once theswitch closes, a resonant circuit is formed from the C-1 capacitor, acharging inductor L1, and a C0 capacitor bank which forms a part ofsolid pulse power system (SSPPS) 404. Current therefore begins todischarge from C-1 through the L1 inductor and into C0, charging up thatcapacitance. Because the C-1 capacitance is much, much larger than theC0 capacitance, the voltage on C0 can achieve approximately 2 times theinitial voltage of that on C-1 during this resonant charging process.The charging current pulse assumes a half-sinusoidal shape and thevoltage on C0 resembles a “one minus cosine” waveform.

[0068] In order to control the end voltage on C0, several actions maytake place. First, the command-charging switch S1 can be opened up atany time during the normal charging cycle. In this case, current ceasesto flow from C-1 but the current that has already been built up in thecharging inductor continues to flow into C0 through the free-wheelingdiode D3. This has the effect of stopping any further energy from C-1from transferring to C0. Only that energy left in the charging inductorL1 (which can be substantial) continues to transfer to C0 and charge itto a higher voltage.

[0069] In addition, the de-qing switch S2 across the charging inductorcan be closed, effectively short-circuiting the charging inductor and“de-qing” the resonant circuit.

[0070] This essentially removes the inductor from the resonant circuitand prevents any further current in the inductor from continuing tocharge up C0. Current in the inductor is then shunted away from the loadand trapped in the loop made up of charging inductor L1, the de-qingswitch S2, and the de-qing diode D4. Diode D4 is included in the circuitsince the IGBT has a reverse anti-parallel diode included in the devicethat would normally conduct reverse current. As a result, diode D4blocks this reverse current which might otherwise bypass the charginginductor during the charging cycle. Finally, a “bleed down” or shuntswitch and series resistor (both not shown in this preferred embodiment)can be used to discharge energy from C0 once the charging cycle iscompletely finished in order to achieve very fine regulation of thevoltage on C0.

[0071] The DC power supply is a 208 V, 90 A, AC input, 800 V, 50 A DCoutput regulated voltage power supply provided by vendors such asUniversal Voltronics, Lambda/EMI, Kaiser Systems, Sorensen, etc. Asecond embodiment can use multiple, lower power, power suppliesconnected in series and/or parallel combinations in order to provide thetotal voltage, current, and average power requirements for the system.The C-1 capacitor in the resonant charger 402 is comprised of two 450 VDC, 3100 μF, electrolytic capacitors connected together in series. Theresulting capacitance is 1550 μF rated at 900 V, providing sufficientmargin over the typical 700-800 V operating range. These capacitors canbe obtained from vendors such as Sprague, Mallory, Aerovox, etc. Thecommand charging switch S1 and output series switch S3 in the embodimentare 1200 V, 300 A IGBT switches. The actual part number of the switchesis CM300HA-24H from Powerex. The de-qing switch S2 is a 1700 V, 400 AIGBT switch, also from Powerex, part number CM400HA-34H. The charginginductor L1 is a custom made inductor made with 2 sets of parallelwindings (20 turns each) of Litz wire made on a toroidal, 50-50% NiFetape wound core with two ⅛″ air gaps and a resulting inductance ofapproximately 140 μH. National Arnold provides the specific core. Otherembodiments can utilize different magnetic materials for the coreincluding Molypermaloy, Metglas, etc. The series, de-qing, andfreewheeling diodes are all 1400 V, 300 A diodes from Powerex, partnumber R6221430PS.

[0072] Once the resonant charger 402 charges up C0, a trigger isgenerated by a control unit (not shown) in the resonant charger thattriggers the IGBT switches S4 to close. Although only one is shown inthe schematic diagram (for clarity), S4 consists of eight parallelIGBT's which are used to discharge C0 into C1. Current from the C0capacitors then discharges through the IGBT's and into a first magneticswitch LS1. Sufficient volt-seconds are provided in the design of thismagnetic switch to allow all of the 8 parallel IGBT's to fully turn on(i.e. close) prior to substantial current building up in the dischargecircuit. After closure the main current pulse is generated and used totransfer the energy from C0 into C1. The transfer time from C0 to C1 istypically on the order of 5 μs with the saturated inductance of LS1being approximately 230 nH. As the voltage on C1 builds up to the fulldesired voltage, the volt-seconds on a second magnetic switch LS2 runout and that switch saturates, transferring the energy on C1 into 1:4pulse transformer 406 which is described in more detail below. Thetransformer basically consists of three one turn primary “windings”connected in parallel and a single secondary “winding”. The secondaryconductor is tied to the high voltage terminal of the primaries with theresult that the step-up ratio becomes 1:4 instead of 1:3 in anauto-transformer configuration. The secondary “winding” is then tied toC2 capacitor bank that is then charged up by the transfer of energy fromC1 (through the pulse transformer). The transfer time from C1 to C2 isapproximately 500 ns with the saturated inductance of LS2 beingapproximately 2.3 nH. As the voltage builds up on C2, the volt-secondproduct of the third magnetic switch LS3 is achieved and it alsosaturates, transferring the voltage on C2 to anode 8 a as shown on FIGS.14A and 14B. The saturated inductance of LS3 is approximately 1.5 nH.Bias circuitry shown in the FIG. 1 at 408 is also used to properly biasthe three magnetic switches. Current from the bias power supply V1,passes through magnetic switch LS3. It then splits and a portion of thecurrent passes through bias inductor L5 and back to the bias powersupply V1. The remainder of the current passes through the pulsetransformer secondary winding and then through magnetic switches LS2 andLS1 and bias inductor L3 back to the bias power supply V1. Bias inductorL2 provides a path back to the power supply from current through thepulse transformer primary to ground. Bias inductors L3 and L5 alsoprovide voltage isolation during the pulse in the SSPPS since the biaspower supply V1 operates close to ground potential (as opposed to thepotentials generated in the SSPPS where the bias connections are made).

[0073] The C0, C1 and C2 capacitances are made up of a number ofparallel, polypropylene film capacitors mounted on a printed circuitboard with thick (6-10 oz.) copper plating. The printed circuit boardsare wedge shaped such that 4 boards make up a cylindrical capacitor deckwhich feeds a cylindrical bus for both the high voltage and groundconnections. In such a way, a low inductance connection is formed whichis important to both the pulse compression and to the stability of theplasma pinch in the DPF itself. The total capacitance for C0 and C1 are21.6 μF each while the total capacitance for C2 is 1.33 μF. The C0 andC1 capacitors are 0.1 μF, 1600 V capacitors obtained from vendors suchas Wima in Germany or Vishay Roederstein in North Carolina. The C2capacitance is made up of three sections of capacitors stacked in seriesto achieve the overall voltage rating since the voltage on the secondaryof the pulse transformer is about 5 kV. The C2 capacitors are 0.01 μF,2000 V de components, again from Wima or Vishay Roederstein. The SSPPSswitches are 1400 V, 1000 A IGBT switches. The actual part number isCM1000HA-28H from Powerex. As noted earlier, 8 parallel IGBT switchesare used to discharge C0 into C1. The SSPPS series diodes are all 1400V, 300 A diodes from Powerex, part number R6221430. Two diodes are usedfor each IGBT switch, resulting in a total of sixteen parallel devices.

[0074] Magnetic switch LS1 is a custom made inductor made with 16 setsof parallel windings (6 turns each) of Litz wire made on a toroidal,ferrite core. The specific core is provided by Ceramic Magnetics of NewJersey and is made of CN-20 ferrite material. The toroid is 0.5″ thickwith an I.D. of 5.0″ and an O.D. of 8.0″. Magnetic switch LS2 is asingle turn, toroidal inductor. The magnetic core is tape wound on a8.875″ O.D. mandrel using 2″ wide, 0.7 mil thick, 2605-S3A Metglas fromHoneywell with 0.1 mil thick Mylar wound in between layers to an outsidediameter 10.94″. Magnetic switch LS3 is also a single turn, toroidalinductor. The magnetic core is tape wound on a 9.5″ O.D. mandrel using1″ wide, 0.7 mil thick, 2605-S3A Metglas from Honeywell with 0.1 milthick Mylar wound in between layers to an outside diameter of 10.94″.

[0075] The pulse transformer is shown at 406, also shown in FIG. 1A hasthree transformer core. Each of the three transformer cores is tapewound on a 12.8 inch O.D. mandrel 422 using 1″ wide, 0.7 mil thick,2605-S3A Metglass from Honeywell with 0.1 mil thick Mylar wound inbetween layers to an outside diameter of 14.65″. Each of the three cores418 are ring shaped, 12.8 inch I.D. and about 14 inch O.D. havingheights of 1 inch. FIG. 1A is an axial cross section sketch showing thephysical arrangement of the three cores and the primary and secondary“windings”. Each of the primary windings actually are formed from twocircular rings 420A and 420B bolted to mandrel 422 and rod-like spacers424.The secondary “winding” is comprised of 48 circularly spaced bolts426. The transformer operates on a principal similar to that of a linearaccelerator, as described in U.S. Pat. No.5,142,166. A high voltagecurrent pulse in the three primary “windings” induce a voltage rise inthe secondary “winding” approximately equal to the primary voltage. Theresult is a voltage generated in the secondary winding (i.e., rods 426)equal to three times the primary voltage pulse. But since the lowvoltage side of the secondary winding is tied to the primary windings afour-fold transformation is provided in this “auto-transformer”configuration.

[0076] Bias inductors L3 and L4 are both toroidal inductors wound on aMolypermalloy magnetic core. The specific core dimensions are a heightof 0.8″, an I.D. of 3.094″, and an O.D. of 5.218″. The part number ofthe core is a-430026-2 from Group Arnold. Inductor l3 has 90 turns of 12AWG wire wound on the toroid for an inductance of ˜7.3 mH while L4 has140 turns of 12 AWG wire wound on it for an inductance of ˜18 mH. Biasinductor L6 is merely 16 turns of 12 AWG wire wound in a 6″ diameter.Bias inductor L4 is 30 turns of 12 AWG wire in a 6″ diameter. Biasinductor L2 is 8 turns of 12 AWG wire in a 6″ diameter. Resistor R1 isan array of twenty parallel resistors, each of which is 27 ohm, 2Wcarbon composition resistor.

Polarity

[0077] In a preferred embodiment of the present invention, theelectrical circuit as shown in FIG. 1 provides positive high voltagepulses to the center electrode 8A as shown in FIG. 2, FIG. 2B1, and FIG.2B2. The direction of current flow of each portion of each initial pulseis shown by arrows 409A, 409B and 409C respectively through the primaryand secondary sides of the transformer 406 and between the electrodes.(The reader should note the direction of electron flow is opposite thedirection of current flow). The reader should note also that during thelatter portion of each pulse the current actually reverses as indicatedby the trace shown at 409D in FIG. 1B so that the voltage on C2 rises toabout +4 kV then rises to about zero.

Reverse Polarity

[0078] In prior art dense plasma focus devices, the central electrode istypically configured as an anode with the surrounding electrodeconfigured as cathode. Thus, the polarity of the electrodes of theembodiment shown in FIG. 2B is consistent with this prior art technique.It is known in the prior art to reverse the polarity of the electrodes;however, the results have typically been a substantial reduction inperformance. (For example, see G. Decker, et al., “Experiments Solvingthe Polarity Riddle of the Plasma Focus,” Physics Letters, Vol. 89A,Number 8, 7 Jun. 1982).

[0079] Applicants have in a preferred embodiment of the presentinvention demonstrated excellent performance by reversing the electrodepolarity of a dense plasma focus device. To do this Applicants modifiedthe circuit shown in FIG. 1 to provide a circuit as shown in FIG. 1D.The basic design of the FIG. 1 circuit made this task relatively easy.The connections on DC power supply 400 were switched, switches S1, S2,S3 and S4 were reversed and diodes D1, D2, D3 and D4 were reversed. Alsothe polarity of bias power supply V1 was reversed. As a result theinitial current flow for each pulse was in the directions shown at 409A,409B, and 409C in FIG. 1D. Thus, the central electrode 8A as shown inthe figures including FIG. 2B2 is initially charged negative and theinitial current flow in this embodiment is from ground electrodes 8B tocentral electrode 8A. The electron flow is in the opposite direction;i.e., from central electrode 8A to surrounding electrode 8B. Anothertechnique for reversing polarity is to modify the pulse transformerdesign to eliminate the “onto” aspect of the transformer. That is toconnect the low voltage side to ground instead of the primary highvoltage. If this is done polarity can be reversed by merely changing thesecondary leads of the pulse transformer. This of course would mean inthis case there would be only a factor of 3 increase in voltage ratherthan 4. But to compensate another primary section could be added.

[0080] Applicants' experiments have demonstrated some surprisingimprovements resulting from this change in polarity. An importantimprovement is that pre-ionization requirements are greatly reduced andmay be completely eliminated. Applicants believe this improvedperformance results from a hollow-cathode type effect resulting from thehollow portion at the top of electrode 8A as shown in FIG. 2A. Accordingto Applicants measurements under various conditions, the quality ofpinches is better than pinches produced with the positive centralelectrode polarity. Applicants estimate increases in EUV output could beup to about a factor of two.

Energy Recovery

[0081] In order to improve the overall efficiency this fourth generationdense plasma focus device provides for energy recovery on apulse-to-pulse basis of electrical pulse energy reflected from thedischarge portion of the circuit. Energy recovery is achieved asexplained below by reference to FIG. 1.

[0082] After the discharge C2 is driven negative. When this occurs, LS2is already saturated for current flow from C1 to C2. Thus, instead ofhaving energy ringing in the device (which tends to cause electrodeerosion) the saturated state of LS2 causes the reverse charge on C2 tobe transferred resonantly back into C1. This transfer is accomplished bythe continued forward flow of current through LS2. After the transfer ofcharge from C2 to C1, C1 then has a negative potential as compared to C0(which at this time is at approximately ground potential) and (as wasthe case with LS2) LS1 continues to be forward conducting due to thelarge current flow during the pulse which has just occurred. As aconsequence, current flows from C0 to C1 bringing the potential of C1 upto about ground and producing a negative potential on C0.

[0083] The reader should note that this reverse energy transfers back toC0 is possible only if all the saturable inductors (LS1, LS2 and LS3)remain forward conducting until all or substantially all the energy isrecovered on C0. After the waste energy is propagated back into C0, C0is negative with respect to its initial stored charge. At this pointswitch S4 is opened by the pulse power control. Inverting circuitcomprising inductor L1 and solid state diode D3 coupled to ground causesa reversal of the polarity of C0 as the result of resonant free wheeling(i.e., a half cycle of ringing of the L1-C0 circuit as clamped againstreversal of the current in inductor L1 by diode D3 with the net resultthat the energy is recovered by the partial recharging of C0. Therefore,the energy which otherwise would have contributed to the erosion of theelectrodes is recovered reducing the charging requirements for thefollowing pulse.

Importance of Output Switch

[0084] As shown in FIGS. 1 and 1D, the pulse power system described inthis invention possesses an output switch that performs severalfunctions. This switch, LS3 in the figure, is a saturable inductor whichwe refer to as a magnetic switch. As explained above, it is biased bybias circuitry 408 so as to initially hold off current flow at thebeginning of each pulse until the inductor saturates at which timecurrent flows for about 100 nanoseconds after which the bias currentre-biases the switch prior to the start of the next pulse which at 5 kHz(for example) comes about 200 micro seconds later. This switch is veryimportant for allowing proper operation of the source at high repetitionrates. Although some EUV sources have been developed without such aswitch, their operation at high rep-rates can be erratic in outputenergy. In these cases, no switch exists between the energy storagecapacitor and the EUV source load. The issue is that the source load maynot fully recover in the short time between the last pulse and the timewhen voltage is applied to the energy storage capacitor in preparationfor the next pulse. At rep-rates of 5 kHz, only 200 μs exists betweenoutput pulses. With many of the other source designs, a significantfraction of this inter-pulse period would be required for charging ofthe energy storage capacitor. Thus, even shorter time may exist betweenthe last pulse generation and the initial voltage application across thecapacitor (and also the load since no output switch exists to isolatethe two). Problems then exist when this time becomes too short for theplasma from the last pulse to cool down and recover (hold off voltageapplication in anticipation of the next pulse). As a result, the sourcemay breakdown again prematurely at lower-than-normal voltages when therecovery is not sufficient. Because the breakdown process is statisticalin nature, there can also be wide variation in the breakdown voltages,leading to large variations in source output EUV energy levels. Thiscauses significant problems in the lithography application since energystability and dose control are very important parameters for processcontrol.

[0085] The advantage of the output switch, LS3 in the inventiondescribed herein, is that it can perform several functions which help toeliminate this issue of premature load breakdown. In the normal pulsegeneration, the LS3 switch acts as a magnetic switch and a diode toprevent current reversal through the load. As a result, any energy notabsorbed by the load is reflected back to the initial storage capacitor,C0 where the energy is recovered and stored for use with the next pulse(as described earlier in the section on Energy Recovery). In thismanner, energy is quickly removed from the load after the main pulsegeneration and is therefore not allowed to continually oscillate untilit is finally and completely dissipated in the load plasma. This helpsto reduce the energy deposited into the load plasma and allows it tobegin the recovery process as soon as possible after the main pulsegeneration and EUV output. In addition, the LS3 output switch providesisolation between the last energy storage capacitor and the source load,allowing the source additional time to recover prior to the next pulsebeing generated. This switch allows the last energy storage capacitorC2, which we refer to as the discharge capacitor, to begin charging assoon as the LS3 switch is reverse biased after the energy recoveryprocess is completed. The design of the bias circuit (including biasinductor L4 and bias power supply VI) can be developed to allow LS3recovery in sufficient time for charging of C2 in the next pulsegeneration sequence at rep-rates of at least 5 kHz. The LS3 switch istherefore initially reverse biased (not conducting in the forwarddirection—towards the load) up until the time when it saturates (as thevoltage on C2 reaches its maximum value). The switch then allows energytransfer from C2 into the load and remains forward conducting until theenergy recovery cycle is completed and reflected energy is recovered allthe way back onto C0. After this period of time, energy from the biascircuit is applied to the main pulse compression circuit and completesthe cycle by reverse biasing the LS3 switch again. Once this isaccomplished, the charging of C2 can take place again without thepotential issue of the load breaking down prematurely (since the LS3switch can now isolate the voltage on C2 from the load).

[0086] As rep-rates for EUV sources may eventually have to extend allthe way to 10 kHz in order to meet EUV source power requirements, theseissues will become even more important since the time between pulseswill become that much shorter.

[0087]FIGS. 1B and 1C show test results from a fourth generation plasmapinch prototype device. FIG. 1B shows the pulse shape on capacitor C2and across the electrodes and FIG. 1C shows a measured photo diodesignal with Xenon as the active gas.

High Temperature Electric Discharge EUV X-Ray Devices

[0088] The high repetition rate reliable, long-life pulse power systemdescribed above can be utilized to provide high voltage electricalpulses to a variety of extreme ultraviolet or x-ray devices. Thesesystem included a dense plasma focus device depicted in FIG. 2A,conventional Z-pinch device shown in FIG. 2B, a hollow cathode Z-pinchdevice shown in FIG. 2C, and a capillary discharge device as shown inFIG. 2D. In each case the light source is generally symmetrical about anaxis referred to as the “Z” direction. For this reason these sourcesespecially the first three are often referred to as “Z” pinch lightsources.

Dense Plasma Focus

[0089] The principal feature of a dense plasma focus EUV device is shownin FIG. 2B. These are anode 8A, cathode 8B and insulator 8C and a highvoltage pulse power source 8D. In this case when high voltage is applieda discharge starts between the cathode and the anode running along theoutside surface of insulator 8C. Forces generated by the high plasmacurrent, forces the plasma generally upward then inward creating anextremely hot plasma pinch just above the center of the anode.

[0090] The parameters specified above for the pulse power system shownin FIG. 1 were chosen especially for this light source to produce 12 Jpulses of about 5,000 volts with pulse durations of about 100 to about500 ns. Preferably a preionizer (which may be a spark gap preionizer) isprovided as described in more detail in U.S. patent application Ser. No.09/690,084 which has been incorporated by reference herein. FIG. 2A(1)shows a cross-section of a portion of a fourth generation plasma pinchEUV light source actually built and tested by Applicants whichincorporates the pulse power system described in FIG. 1. Many of theelectrical components referred to above are designated in FIG. 2A(1).FIG. 2A(2) is a blow-up of the electrode region of the device showing ingreater detail the anode 8A, the cathode 8B and the spark gappreionizers 138. FIG. 2A(3) is a drawing of the fourth generation deviceshowing many of the electrical components shows in FIG. 2A(1) and alsoshowing vacuum 3.

Conventional Z Pinch

[0091] A conventional Z-pinch light source is shown in FIG. 3. In thiscase the discharge starts between the anode and the cathode along theinside surface of insulator 9C. The forces generated by thehigh-current, forces the plasma to the center of the cylindrical volumeformed by insulator 9C and causes the plasma to pinch with extremely hottemperatures near the upper end of the volume.

[0092] The pulse power circuit shown in FIG. 1 with the componentsdescribed above would work for embodiments of the conventional Z-pinchdesign, although persons skilled in the art may choose to make changesto coordinate the parameters of the pulse power electrical componentswith specific design parameters of the Z-pinch. For example, if 5,000volt pulses are preferred this can be easily accomplished simply withthe same basic circuit as shown in FIG. 1 but with one additionalone-turn primary winding on the pulse transformer 406,.With this designa preionizer is usually provided to help initiate the plasma at thestart of each pulse. These preionizers may be spark gap or otherpreionizer source and are usually powered from a separate source notshown.

Hollow Cathode Z-Pinch

[0093] The hollow cathode Z-pinch shown in FIG. 2C is very similar tothe conventional z pinch. The difference being that the cathode isconfigured to produce a hollow below the cylindrically shaped insulator.This design can avoid the need for a preionizer because a very largenumber of ions and electrons are naturally produced near the top of thehollow region 9E at the beginning of each pulse when the high voltageincreases to a sufficiently high level. For this reason this design doesnot require a high voltage switch to initiate the discharge. Thedischarge is referred to as having been self-initiated.

[0094] When using the power supply shown in FIG. 1 to provide pulsepower for this design, the last saturable inductor L53 could beeliminated or its value reduced substantially since the development ofplasma in the hollow in the cathode serves the same purpose as saturableinductor L53 of holding off the discharge until the peaking capacitor C2is sufficiently charged, then permitting current to flow substantiallyunimpeded.

[0095] This hollow cathode Z-pinch may be designed for significantlyhigher pulse voltages than the first two designs. This is no problemwith the power supply shown in FIG. 1. A discharge pulses of, forexample, 10,000 Volts are easily provided by merely increasing thenumber of one-turn primary windings of the transformer 406 from 3 to 9.

Capillary Discharge

[0096] A drawing of a conventional capillary discharge EUV light sourceis shown in FIG. 2D. In this design the compression of the plasmacreated by the high voltage discharge between the cathode and the anodeis achieved by forcing the plasma through a narrow capillary whichtypically has a diameter in the range of about 0.5 mm to 4 mm. In thiscase the pulse duration is in the order of about 0.5 microseconds to 4microseconds as compared to about 100 to 500 nanoseconds for theembodiment shown in FIGS. 2, 3 and 4. Also, the pulse voltages aretypically substantially lower, such as about 1500 volts. However, thesame pulse power system provides an excellent electrical power sourcewith minor modifications. A simple modification is to eliminate the laststep of magnetic compression which is accomplished by leaving off the C2capacitor bank and the LS3 saturable inductor. The peak pulse voltagecould be reduced to 2,000 by windings in pulse transformer 406 fromthree to one, or the transformer could be eliminated with an increase inthe initial charging voltage to provide electrical pulses of a fewmicroseconds and a peak voltage of about 1500 volts.

Laser Produced Plasma

[0097] As described in the background section of this specification, aprior art technique for producing extreme ultraviolet light on softx-rays is to use short pulse lasers to produce a very hot plasmas whichare similar to the plasmas produced in the plasma pinches describedabove. Prior art techniques typically utilize solid state lasers such asQ-switch Nd-YAG lasers pumped with diode lasers (or flash lamps) toproduce very high power nano-second or pico second laser pulses whichare focused on a target material which may be the same target materialsas the active materials identified above such as lithium and tin whichproduce debris or xenon which does not produce debris. Some of theseprior art light sources are described in U.S. Pat. Nos. 5,668,848,5,539,764, and 5,434,875, all of which are incorporated herein byreference. These prior art patents teach the use of an Nd-YAG laser forgenerating the plasma and the use of an Nd-YAG seeded XeCl excimerpre-amplified and an XeCL excimer amplifier for producing the high power(such as 1×10¹¹″ Watts) very short pulse laser beam for generatingplasmas in target material. Other laser systems (including excimer lasersystems) have been proposed for producing x-rays (see for example, M.Chaker, et al., J. Appl. Phys.63, 892 (1988; R. Popil et al, Phys. Rev.A 35, 3874 (1987); and F. O'Neill et al., Proc. SPIE 831, 230 (1987).Applicants have determined that many of the novel features developed byApplicants in connection with Applicants' development of their plasmafocus devices can be applied with respect to laser produced plasmas justas well as plasmas produced by the various pinch devices shown in FIGS.2A-D.

[0098] Applicants' employer is the leading supplier in the United Statesand internationally of excimer laser light sources for integratedcircuit lithography. These lasers are KrF excimer lasers operating at248 nm and ArF lasers operating at 193 nm. These lasers are extremelyreliable, typically operating 24 hours per day 365 days per year withup-times on the average better than 99 percent. During the past severalyears pulse repetition rates of these lasers have increased from about100 Hz in 1990 to 4000 Hz in 2003. The average power of these lasers hasincreased from about 1 Watt in 1990 to about 120 Watts in 2003. Thepulse duration is about 20 ns and the current pulse energy is about 30mJ. Techniques to increase repetition rates of these lasers to 6,000 to10,000 Hz are described in U.S. patent application Ser. No. 10/187,336also incorporated herein by reference.

[0099] Applicants believe that the excimer laser systems currently inuse as the leading lithography light source at 248 nm and 193 nm can beadapted to provide extreme ultraviolet light in the range of 11 to 14nm. Examples of these laser systems are described in the following U.S.patents and patent applications which are incorporated herein byreference: U.S. Pat. Nos. 6,128,323; 6,330,261, 6,442,181, 6,477,193 andU.S. patent application Ser. Nos. 09/854,097, 09/943,343, 10/012,002,10/036,676, and 10/384,967.

[0100] In a laser-produce plasma light source the laser energy isabsorbed by the inverse Bremsstrahlung mechanism. Due to their shorterwavelength, excimer lasers can couple energy more efficiently to thetarget plasma than near infrared or visible laser radiation from(frequency-doubled) solid state lasers. (The plasma frequency and thusthe critical density is higher at shorter wavelength of the pump laser.)Due to their shorter wavelength, excimer lasers can (if desired) befocused more tightly to a (diffraction-limited) spot size than longerwavelength (e.g., solid-state) lasers. This increases the power densityof the source. The excimer laser should be a Cymer laser, since theseare the most reliable ones in the world. If desired several excimerlaser beams can be combined in one spot. This permits power scaling.

[0101] One or several excimer laser beams are tightly focussed onto a(gaseous, liquid or solid) target inside a vacuum chamber to generate ahot laser-produced plasma. When the proper target material is used andthe right mean electron temperature is reached in the plasma, EUVradiation at 13.5 nm can be efficiently generated. Suitable targetmaterials are xenon, tin and lithium. Xenon has advantages with respectto lower debris production. Unfortunately, xenon is not the mostefficient target at 13.5 nm, in particular not for a laser-producedplasma. It produces radiation much more efficiently at around 11 nm. Oneof the best target configurations is a liquid jet of xenon, since theplasma can be generated at a fairly large distance from the nozzle. Tinhas advantages with respect to conversion efficiency, since manyionization stages contribute simultaneously to the 4d-4f emission atabout 13.5 nm. Indium has advantages, if its corresponding radiationband at 14 nm and above is used. (There, the manufactured multi-layermirrors have only slightly lower peak reflectance but larger bandwidth.Therefore, a higher integral in-band intensity can be obtained.) Lithiumhas advantages in case a light source with narrower emission bandwidthshould be required, since lithium emits efficiently in a narrow line at13.5 nm. It may be advantageous to use a small cavity for confinement,in particular, if metals are used as laser targets. Liquid metal targets(molten tin, indium or lithium) offer the possibility of high targetdensity and reproducible target conditions when the source is operatedat constant repetition rate. (A crater will be formed, but a given,fairly constant shape will dynamically evolve at a given repetition ratebetween the laser pulses.)

[0102] The excimer pump laser should preferably be operated withkrypton-fluoride at 248 nm, since this is the most efficient excimerlaser and since associated optics issues for the focussing optics areless severe. The excimer laser preferably is operated broad-band and ina MOPA configuration, since a very high output power is needed. Thelaser pulse duration should be as very short (a few nano seconds such asabout 20 ns), since it has to be matched to the plasma expansion time.The peak power will be high. In preferred embodiment the laser isoperated at repetition rates of 10 kHz or higher, at least at more than5 kHz. To increase the effective repetition rate, one may also combineseveral lasers operated at suitable different times in the interval.This depends to a large part also on the target configuration andreplenishing rate of the target material. It is advantageous to have atailored laser pulse that is incident on the target. In a preferredembodiment a pre-pulse portion generated for instance by the excimerlaser oscillator (which may bypass the power amplifier in order tominimize the travel time to the target) containing up to a few percentof the total laser energy arrives at the target first to form apre-plasma. This pre-plasma will absorb the main laser pulse much moreefficiently. The pre-plasma can also be accomplished by using adifferent, perhaps smaller-power laser.

[0103] The laser beam will be focused by optimized focusing opticsmounted immediately in front or behind of a vacuum window. The objectiveis to achieve a focal spot of less than about 100 μm diameter. The spotsize depends to some extent on the laser pulse duration (10 to 30 ns),since the plasma expansion velocity has to be taken into account. Inshort, the laser pulse duration has to be short enough and the spot sizesmall enough to keep a large portion of the plasma tightly togetherduring the main heating period. Typical expansion times are on the orderof 10-100 μm per nanosecond. The laser systems described in details inthe above identified excimer laser patents and patent applicationsproduce a very line-narrowed pulsed laser beams, line narrowed to about0.5 μm or less. This permits focusing to a quarter micron spot. However,these laser system can be operated broadband in which the bandwidth ofthe output pulse laser beam is about 35 nm for KrF lasers with the linecenter at about 248 nm. Broadband operation permits substantialincreases in energy of the output beam. For example, a KrF MOPA systemof the type described in patent application Ser. No. 10/384,967 couldproduce 330 mJ pulses (as compared to the 30 mJ line narrowed pulses).The instantaneous pulse power for the 20 ns pulses is about 165×10⁶Watts. According to experiments performed at Lawrence LivermoreLaboratories (J. Appl. Phys. 79(5): March 1996) using a Nd/YAG laser,the maximum EUV output occurred at a laser intensity of 2×10¹¹″ W/cm².While maximum conversion efficiency (EUV energy output/laser energyinput) occurred at about 2×10¹¹ ″ W/cm². These experiments indicated notmuch variation in results with changes in wavelength. The pulse durationin the experiments were not much different from the 20 ns pulses ofApplicants employers' excimer lasers. For the 165×10⁶ Watt pulsestherefore Applicants prefer spot sizes in the range of about 0.1 mm²which would provide intensities of about 1.6×10⁻¹¹ W/cm² which is inbetween maximum efficiency and maximum output.

[0104] The energy of the laser pulse is about 330 mJ so at a conversionefficiency of about 0.006 the EUV pulse energy is about 2 mJ/pulse. At6000 Hz this corresponds to an EUV production of about 12 Watts. About20 percent of this light can be collected and delivered to anintermediate focus such as location 11 in FIG. 19 using technologydescribed herein. So the average in-band EUV power from one excimerlaser produced plasma delivered to the intermediate focus is about 2.4Watts. The combination of two systems would produce about 5 Watts. Insome applications, this is sufficient.

[0105] Applicants have been told that makers future EUV lithographymachines have desires for an EUV light source of about 45 Watts to about100 Watts at an intermediate focus such as location 11 in FIG. 19. Butthis requirement is for several (at least 5) years in the future and therequirement is contingent on development of corresponding lithographysystems that can handle EUV power in the 100 Watt range. Since theexcimer laser can be expected to couple energy more efficiently to theplasma (shorter wavelength, higher critical density) than a solid-statelaser driver at 1.06 μm, the conversion efficiency should be higher forthe excimer-laser produced plasma as compared to the prior art NdYAGlaser.

[0106] About 10 kW of laser power will be needed to generate therequired EUV power of about 100 Watt at the intermediate focus of thelithography tool. With expected improvements in demonstrate conversionefficiencies, each KrF module (broad-band operation at 248 nm) can beexpected to provide about 1 to 1.2 kW of laser power (e.g. 6 kHzrepetition rate operation at 200 mJ/pulse). A total of nine such moduleswould deliver the required laser power. More than 200 W of in-band EUVradiation would be produced at the source (2% bandwidth into 2π) andabout 100 W in-band EUV could be collected and delivered to theintermediate focus.

[0107] There are different ways to combine the laser beams(multiplexing). Laser beams can be (nearly) overlapped optically bymirrors and lasers beams can be focused through the same lens fromslightly different directions onto the same focal spot. The lasers canalso be triggered in a staggered fashion such the effective repetitionrate is increased, provided the target is replenished fast enough thatit can sustain the high repetition rate. For instance, tripling of therepetition rate with three laser systems to about 18 kHz seems feasible.

[0108]FIG. 4 shows one embodiment where the laser beams from severallaser modules can be aimed at different portions of the focusing lensand made to spatially overlap in the common focus which corresponds tothe location of the laser-produced plasma. The emitted EUV radiation iscollected over a large angular range by the multi-layer coated firstcollector mirror and directed to the intermediate focus.

[0109]FIG. 4A shows another embodiment where the laser beams fromseveral laser modules are overlapped in a common laser focus withseparate focusing optics for some of the laser beams. The laserradiation can be focused through several openings in the first collectormirror. This embodiment makes use of the fact that the EUV radiationgenerated from the laser plasma has an angular distribution that ispeaked to some extent in the direction of the incoming laser beam (andweaker at angles orthogonal to the laser beam). In this embodiment, theregions of strongest emission are not blocked by the space required forthe beam delivery device.

Target Delivery

[0110] The preferred target for the laser plasma is a so-calledmass-limited target. (Just the right amount necessary for thelaser-produced plasma is provided, no more, in order not to increase theproduction of debris unnecessarily. For xenon, a preferred targettechnique is a thin liquid jet. Cluster beam targets and spray targetsmay also be employed using erosion resistant nozzles. For metals (tinand indium), liquid metal drops, immersed in a helium beam, aresuitable. A nozzle, mounted from the top, and a target beam dump mountedbelow, comprise a suitable system. See FIG. 4B. The plasma-facingsurfaces may be coated by a thermally conductive thin film, like carbonor diamond coating, to reduce erosion, since ion sputtering is reduced.

Laser Plasma Supported EUV Pinch

[0111] The laser plasma source has the advantages of high sourcebrightness (small source volume), no erosion, less debris generation. Ithas the disadvantages of high cost-of-ownership and inefficient totalenergy conversion balance. The discharge source has the advantages ofdirect coupling of the electrical energy into the pinch plasma and ofsimplicity. It has the disadvantages of electrode erosion and highdebris production, as well as thermal management issues.

[0112] The laser beam(s) and the laser plasma are used to define theplasma geometry, discharge pathways and plasma pinch location. Thearrangement is such that there is a larger distance from the electrodesto the plasma focus than in a pure discharge source. This reduces thepower density at the electrode surfaces, since they can be large, andthus also electrode erosion, debris generation and thermal managementrisks are reduced. On the other hand, the main power input is providedby the low-inductance electrical discharge. This ensures a much moreefficient energy coupling to the plasma than would be available for apure laser plasma source. The arrangement of the electrodes is morespherical than for a conventional Z-pinch. This and the laser-plasmainitiation increase the source stability. The timing of pre-ionization,laser plasma generation and main pinch plasma generation givesadditional control for optimization of the production of EUV radiation.

[0113] The device is mainly a discharge-produced EUV light source thathas the additional benefits of laser-plasma supported dischargeinitiation. The electrodes can be connected to the same pulsed-powersystem that is used presently (and in the future) for the DPF machines.(10 J to 20 J delivered pulse energy, 30-100 ns pulse length, repetitionrate of several kHz, peak voltage of several kV, peak current severaltens of kA.) The inner electrode can be charged positive or negative.The outer electrode is at ground potential. As shown in FIG. 4C theelectrode arrangement is somewhat different from the DPF arrangement.The (water-cooled) electrodes are bigger and the electrode surfaceinvolved in the discharge is bigger. It is on the order of 30 to 50 cm².There is an insulator disk between the electrodes to prevent a dischargealong the direct line-of-sight.

[0114] There is a means of pre-ionization, e.g., pulsed RF-pre-ionization via RF-coil. The pulsed laser beam (excimer laser orsolid-state laser) that propagates on-axis is focused by a focusingoptics into the center of the arrangement to a focal spot with adiameter of ca. 100 μm. The laser can be a KrF-broad-band excimer laserwith 100 mJ to 200 mJ pulse energy, about 10 to 15 ns pulse length andseveral kHz repetition rate. There could also be several laser beamsfocused into a common spot in the center of the arrangement. The targetgas, xenon or a mixture of xenon and helium, is inserted from inside ofthe inner electrode and is pumped away by a vacuum pump. Typicaloperating pressure is in the range of 1 to 0.01 Torr. The discharge canbe operated on the left side of the Paschen curve. If the innerelectrode is pulsed-charged by a negative high voltage, it can beconfigured as a hollow cathode.

[0115] First, RF pre-ionization is triggered to enable easy breakdown ofthe low-density gas. Next, the laser beam arrives and generates awell-defined plasma spot at the center of the arrangement. The gasbreaks down near the laser focus, since it was pre-ionized. Then themain discharge from the pulse-compression circuit is applied. A pinchwill develop on-axis at the laser-plasma spot. Pinching occurs bymagnetic self-compression. The laser-plasma spot defines the location ofthe pinch and increases its positional stability. (In case theinductance in the center should be too high, the laser beam needs to bedoughnut-shaped in order to provide a discharge channel. This has to betested experimentally.) The expanding shock front from the laser plasmawill encounter the radial compression front from the main pinch plasmawhich is stronger. A pinched plasma channel develops which will heat thegas to high ionization levels that will emit the EUV radiation. Thecounter-propagation of the two plasma shock fronts can effectivelyincrease the duration of the pinch and thus the duration of the EUVemission. The EUV radiation is emitted in all directions. The radiationemitted through the large opening of the outer electrode can becollected by grazing-incidence collection optics. The energy, the sizeof the focus and the timing of the laser plasma determine the size ofthe main pinch plasma.

Radiation Collector Materials

[0116] The radiation produced at the radiation spot is emitted uniformlyinto a full 4π steradians. Some type of collection optics is needed tocapture this radiation and direct it toward the lithography tool.Several materials are available with high reflectivity at small grazingincident angles for 13.5 nm UV light. Graphs for some of these are shownin FIG. 11. Good choices include molybdenum and rhodium in the range of0 to about 20 degrees and tungsten for very small grazing angles. Thecollector may be fabricated from these materials, but preferably theyare applied as a coating on a substrate structural material such asnickel. This conic section can be prepared by electroplating nickel on aremovable mandrel.

Conical Nested Collector

[0117] To produce a collector capable of accepting a large cone angle,several conical sections can be nested inside each other. Each conicalsection may employ more than one reflection of the radiation to redirectits section of the radiation cone in the desired direction. Designingthe collection for operation nearest to grazing incidence will produce acollector most tolerant to deposition of eroded electrode material. Thegrazing incidence reflectivity of mirrors such as this depends stronglyon the surface roughness of the mirror. The dependence on surfaceroughness decreases as the incident angle approaches grazing incidence.Applicants estimate that their devices can collect and direct the 13 nmradiation being emitted over a solid angle of least 25 degrees.

[0118] In another preferred embodiment the collector-director isprotected from surface contamination with vaporized electrode materialby a debris collector which collects all of the tungsten vapor before itcan reach the collector director 4. FIG. 9 shows a conical nested debriscollector 5 for collecting debris resulting from the plasma pinch.Debris collector 5 is comprised of nested conical sections havingsurfaces aligned with light rays extending out from the center of thepinch site and directed toward the collector-director 4.

[0119] The debris collector collects vaporized tungsten from thetungsten electrodes and vaporized lithium. The debris collector isattached to or is a part of radiation collector-director 4. Bothcollectors may be comprised of nickel plated substrates. The radiationcollector-director portion 4 is coated with molybdenum or rhodium forvery high reflectivity. Preferably both collectors are heated to about400° C. which is substantially above the melting point of lithium andsubstantially below the melting point of tungsten. The vapors of bothlithium and tungsten will collect on the surfaces of the debriscollector 5 but lithium will vaporize off and to the extent the lithiumcollects on collector-director 4, it will soon thereafter also vaporizeoff. The tungsten once collected on debris collector 5 will remain therepermanently.

Parabolic Collector

[0120]FIG. 7 shows the optical features of a collector designed byApplicants. The collector is comprised of five nested grazing incidentparabolic reflectors, but only three of the five reflections are shownin the drawing. The two inner reflectors are not shown. In this designthe collection angle is about 0.4 steradians. As discussed below thecollector surface is coated and is heated to prevent deposition oflithium. This design produces a parallel beam. Other preferred designswould focus the beam. The collector preferably is coated with a materialsuch as those referred to above and graphed in FIG. 11 possessing highglazing incidence reflectivity in the 13.5 nm wavelength range.

Ellipsoidal Mirror

[0121] Another collector-director designed to focus the beam is shown inFIG. 7B. This collector-director utilizes an ellipsoidal mirror 30 tofocus the EUV source. Mirrors of this type are available commerciallyfrom suppliers such as Reflex S.V.O. with facilities in the CzechRepublic and are distributed in the United States by Bede ScientificInstruments Ltd. with offices in the United Kingdom and Englewood, Colo.The reader should note that this mirror collects only rays at anglesshown at 32 in FIG. 7B.

[0122] However, additional mirror elements could be included insidemirror 30 and outside mirror 30 to collect and focus additional rays.The reader should also note that other mirror elements could belocalized downstream of mirror 30 to collect the narrow angle rays orupstream of mirror 30 to collect the wider angle rays.

Tandem Ellipsoidal Mirror

[0123]FIG. 19 shows a preferred collector director design for greatlyimproving the EUV beam profiled. This is a tandem ellipsoidal mirrorunit which collects and directs the EUV radiation produced in the plasmapinch.

[0124] In most lithography applications the target region needs to beexposed uniformly. A single or nested ellipsoidal mirror of the typeshown in FIG. 2A when used to collect and re-focus the EUV radiationproduces a very non-uniform annulus of radiation upstream and downstreamof focus spot 11 shown in FIG. 2A. This is a natural effect caused bythe geometry of the ellipsoidal collector. The front of the mirrorcollects a greater solid angle of the source emission per unit mirrorsurface area than the back of the mirror. This effect can be reversed byusing a second ellipsoidal mirror 44 in tandem with the first mirror 42as shown in FIG. 19. (In this embodiment, single ellipsoidal mirrors areused without a second nested ellipsoidal mirror.) The second ellipsoidalmirror 44 is a mirror image of the first ellipsoidal mirror 42“reflected” about the second focal point of the first mirror. Thisplaces the second ellipsoidal mirror on the same optical axis as thefirst mirror so that its first focal point is at the second focal pointof the first mirror. In this case of the tandem ellipsoidal mirror theradiation leaving the second focal point of the second mirror is annularbut the radiation within the annulus is uniform. The exposure uniformityis now a function of the surface figure of the ellipsoidal mirrors andnot the inherent collection geometry of the ellipsoidal mirror.

Analysis

[0125] The optical characteristics of the tandem ellipsoidal mirror wereanalyzed by Applicants with the ray tracing code, TracePro, supplied byLambda Research Corporation of Littleton, Mass. The EUV radiation fromthe DPF source is incoherent. Consequently, a ray tracing code can beused to determine the properties of the radiation collected and leavingthe tandem mirror. The EUV radiation requires special reflectivesurfaces such as molybdenum or ruthenium. This analysis was performedunder the assumption that the mirror surface has a perfect ellipsoidalreflector and that the radiation is not polarized during reflection. Themirror surface was assumed to be pure ruthenium reflecting at 13.5 nm.Also, the source has been assumed to be a 50 micron diameter disc andthat the radiation emits isotropically from each point on its surface.These assumptions do not detract from the basic ability of the tandemmirror to produce a uniform annular exposure region.

[0126] The geometry of the tandem ellipsoidal mirror is illustrated inFIG. 19. Both mirrors have the same parameters. Their minor radius is 40mm and their focal length is 150 mm. The mirrors are each 100 mm longand have been cut through their minor diameter. The figure also shows afew random rays collected by the first mirror. A fraction of theradiation that leaves the plasma pinch source 46 at the first focalpoint of the first mirror is collected and re-focused at the secondfocal point 11 of the first mirror. The radiation leaving focal point 11at 300 mm from source 46 is collected by the second ellipsoidal mirrorand re-focused at the second focal point of the second mirror 48 at 300mm from focal point 11. At focal point 48 a 1:1 image of the source isproduced. As the radiation leaves focal point 48, the rays diverge toproduce an annular exposure area at detector 50 which is located 9 mmfrom focal point 48. The intensity in this annular region is uniform asshown by the TracePro calculation in FIG. 19. The uniformity in the mainannular region is within ±2.5% of the mean value. A simulation performedby Applicants of the beam profile at detector 50 is shown in FIG. 19which may be compared with a similar simulation made for the beam crosssection at 9 mm downstream of focal point 11. A cross section of the twoprofiles is compared in FIG. 19 with the detector 50 cross section shownat 52 and the cross section of the FIG. 19 beam profile at 54.

Fabrication

[0127] The techniques for ellipsoidal mirror fabrication have beenimproved over the past few 10s of years. The surface quality of thesemirrors can now be made to satisfy the requirements of surface figure,surface roughness, and the material of the reflecting surface for theiruse in the EUV region. Four materials have been identified as possiblecandidates for the EUV ellipsoidal mirror surface: molybdenum,ruthenium, rhodium, and palladium. These materials have relatively highgrazing incidence reflectivity at 13.5 nm. The grazing incidencereflectivity must remain high at relatively high angles to allow themirror to collect a reasonable solid angle subtended from the source.Theoretically, ruthenium has the highest collection efficiency of thefour materials listed.

[0128] These mirrors are fabricated though a series of processes. First,a mandrel is made that has the outside figure of the desired mirror.Typically, the mandrel is made undersize using aluminum and then coatedwith electroless nickel containing 15% phosphorus to make the mandreloversize. The electroless nickel is put on about 0.5 mm thick so thatthe entire surface can be diamond turned to the desired mirror surfacefigure by vendors such as Corning Netoptic with offices in Marlborough,Mass. This typically leaves about 0.1 mm of nickel on the mandrelsurface. Although the present technology of diamond turning is very goodthe surface at this stage is not adequate for use as an EUV mirror. Thediamond turning can be accurate enough for the figure requirements thatinclude the deviations from the elliptical surface front-to-back and theroundness of the surface but the micro-roughness is too high. Thediamond turned surface must be polished to reduce the micro-roughness toless than 0.5 nm RMS. The hardness of the nickel surface imparted by thehigh phosphorus content of the electroless nickel is required for thehigh degree of polishing. After the electroless nickel surface isadequately polished and the surface figure is within specifications, thereflecting surface material is coated onto the mandrel surface. Theexact procedure used to coat the surface is dictated by the propertiesof the reflecting material being added to the surface. After thereflecting coating has been placed on the mandrel, nickel iselectroformed over this surface to a thickness of about 0.5 mm. Theelectroformed nickel is removed from the mandrel by applying force alongthe axis of the mandrel between the mandrel and the electroformednickel. The reflecting surface stays with the electroformed nickel shellto form the mirror as it slides off the nickel surface on the mandrel.The surface of the highly polished electroless nickel with the highphosphorus content acts as a natural release agent for the reflectingsurface. After the mirror has been removed from the mandrel and themandrel re-polished, the mandrel is then available to make additionalmirrors that are exact copies of the first mirror.

Alignment

[0129] The positioning of the mirrors relative to the source and to eachother is critical to the correct function of the tandem ellipsoidalmirrors. Alignment can be accomplished on an optical bench with a sourceplaced at the same location as the DPF EUV source. One must takeadvantage of the optical properties of these ellipsoidal mirrors. If adetector plane is placed perpendicular to the optical axis near thesecond focal point, the small source, 50 microns diameter, e.g., can beplaced near the first focal point of the ellipse. The image will only becentered and symmetric if the detector is at the second focal point.After the axial location of the second focal point has been determined,the detector array can be moved away from the focal point. Now the imagewill only be symmetric if the source is on the mirror axis. Thisrequires positioning the source in two spatial dimensions. The axiallocation of the first focal point can be determined by moving thedetector to the second focal point and then moving the source along themirror axis until the detector gives a maximum signal in the imagecenter.

[0130] This procedure must be repeated for the second mirror. After thetwo mirrors have been aligned, the entire assembly must be transferredto the DPF. The fixture must be adequately keyed to place the EUV sourceat the first focal point of the first mirror. The accuracy ofpositioning must be at least 25% of the effective diameter of the DPFEUV source. The present estimate of the DPF source diameter is 80microns while looking along the machine axis. Hence, the expectedalignment accuracy is 20 microns in the plane perpendicular to themachine axis. The axial alignment of the tandem mirror is not ascritical and is expected to be about 0.5 mm.

Lithography Projection Optics

[0131] The EUV projection in preferred embodiments is designed to mapthe source spot into the entrance pupil of the projection optic and tomap the far field intensity (i.e. the energy vs. angle) of the sourceonto the reticle. Such designs are desirable because the uniformity inthe entrance pupil, though important, is not critical while theuniformity at the reticle plane is critical. This design conceptexploits the fact that the emission is isotropic and thus has uniformintensity vs. angle. The dual mirror concept restores this uniformintensity vs. angle property (at least within the cone of capture anglefor the mirrors). The EUV illuminator take the “ring” of intensityversus angle, break it into pieces or arcs, and overlay these arcs ontothe reticle. This further improves the uniformity and can be done in EUVsystems since they are scanners and thus require illumination only overa slit region.

Debris Mitigation

[0132] Both the mid-focal point 11 between the two mirrors and the finalfocal point 48 allow the DPF source region to be isolated from thelithography exposure region. At these points the EUV radiation can passthrough pinholes that block any source debris or active gas (thatpenetrated into the region of the first elliptical mirror unit) fromreaching the exposure chamber but not the EUV radiation. Also, thesesmall pinholes allow the exposure chamber to have a much lower pressurethan that required for DPF operation.

Hybrid Collection

[0133] Based on currently available reflector technology, only two typesof reflectors exist which provide reflection values in the 0.7 orgreater range for this 12-14 nm EUV light. As shown in FIG. 11 a fewmaterials provide good grazing angle reflectors. For example, reflectionfrom smooth molybdenum surfaces is about 90% grazing angles less than 10degrees, but reflection from molybdenum drops rapidly at grazing anglesgreater than 15 degrees to less than 10% at 25 degrees. On the otherhand, special multi-layer reflectors have been designed that providereflectivity values in the range of 60% to 70% at normal incident anglesbut the reflectivity ofthese multi-layer reflectors remains high foronly about 5-8 degrees from normal and drops to less than about 10% atincident angles greater than about 10 to 15 degrees. Other multi-layermirrors can be designed for about 30 percent reflectivity over a broaderrange up to about 20 degrees around normal. Using these available mirrortechnologies Applicants have developed various collector designs formaximizing the collected light. Three of these designs are shown inFIGS. 11B, 11D and 11E. Applicants refer to these collectors as hybridcollectors since they utilize multiple collection designs. For example,the prior art includes nested elliptical mirrors and nested grazingangle by hyperbolic mirrors including double bounce hyperbolic mirrorsand most multi-layer reflector designs are single bounce near normalhyperbolic designs. FIG. 11B is a partial cross-section of a hybridcollector utilizing two ruthenium coated ellipsoidal mirrors 80 and 81and two double bounce ruthenium coated parabolic mirrors 82 and 83 toprovide a 1500 mm focal length. FIG. 11C shows the reflectionefficiencies of the mirrors at the angles of incident of the lightbetween about 10 degrees and 55 degrees. This design collectssignificantly more light than prior art elliptical designs or prior arthyperbolic designs. Applicants estimate that about 25 percent of theemitted light is collected and 79 percent of the collected light isdelivered to the intermediate focus at 1500 mm. This equates to anestimate 20 percent collection efficiency.

[0134]FIG. 11D shows a modified version of the FIG. 11B collector inwhich an additional parabolic double reflection mirrors 84 and aparabolic triple reflection mirror 85 are utilized to increase the netenergy collected to about 28 percent.

[0135]FIG. 11E shows a third hybrid version also a modification to the11B collector which (in addition to the two ellipsoidal reflectors) andthe two-bounce parabolic reflectors, Applicants have added a thirdtwo-bounce parabolic mirror 86 and a grazing angle curved ray-tracedmirror 87 and a multi-layer parabolic mirror 88 reflecting at about 9degrees from normal to increase the collection efficiency from about 20%to about 25%.

[0136] In another embodiment, a multitude of laser beams can be focusedthrough corresponding openings of the electrodes to a common centralfocal spot. The main discharge follows along the laser channels andconverges onto the center plasma.

Debris Shields Techniques for Making Debris Shield

[0137] As described above debris shields are important elements insubstantially all EUV light sources now under consideration. The perfectdebris shield won't trap all debris and transmit all in band radiation.Since the debris shield will likely have a limited lifetime, it shouldalso preferably not be difficult to make. Three preferred techniques forfabrication debris shields are shown in FIGS. 28A-B, 29A-C and 30A-C.For the technique described in FIGS. 26A and B, removable skinny pyramidshaped forms as shown in FIG. 26A are fabricated and the small end ofthe forms are inserted in a grid shaped structure such as the one shownin 28B. A spacer plate with tabs matching a hole at the large end ofeach of the forms is placed over the larger end of the forms to separateeach form from other forms by the thickness of the grid which preferablyis about 0.01 to 0.1 mm or less. The grid spacing provides a narrowspace between the forms which is filled with a liquid metal or liquidceramic. When the metal or ceramic has hardened the forms are removed tocreate the debris shield. For FIGS. 5A-C technique, hollow cones such asthose shown 76 in FIG. 5B are welded from very thin about (0.1 mm) metalfoil cut from foil sheets as shown at 77 in FIG. 5A. These hollow conesare inserted into a metal form as shown at 78 in FIG. 5C to form thedebris shield.

[0138] As shown in FIGS. 7A-C, a preferred debris shield can be made bylaminating thin sheets. Each sheet has its own individual radial grillework with grille work patterns growing larger for each sheet so thatwhen multiple sheets are stacked the desire shape is produced as shownin FIGS. 7A-C. An advantage of the laminated approach is that the unevensurfaces of the channels provides a torturous path for particulate withmultiple eddys for particulate to collect within. Another advantage isthat the shield assembly can be constructed of multiple materials. Itmay prove beneficial to use heat resistant ceramics close to the lightsource, or perhaps materials with excellent thermal conductivity such ascopper that can assist in removing heat from the same region.

Magnetic Suppression

[0139] Another technique for increasing the effectiveness of debrisshields in these EUV light sources is to apply a magnetic field in theregion of the debris shield and the region between the pinch and theshield. The magnetic field preferably is directed perpendicular to theaxis of the EUV beam so as to force charged particles into a curvedtrajectory as it approaches and passes into the debris shield. Toenhance the effectiveness of the debris shield the debris can be furtherionized post pulse. This can be done with the same components used forpreionization or similar ionization components could be used for thepost pinch ionization.

[0140] In another embodiment a coil with large diameter (larger than thecollector mirror diameter) will be mounted co-axially with the mirrorand plasma source. Gneerally, a high current will be applied to the coilto induce a high magnetic field in the axial direction. Preferentially,the current may be pulsed (pulse width on the order of several 10 μs) toachieve a high induction field strength (on the order of 10 Tesla).Constant fields and preferentially super-conducting coils may also beemployed to generate these high fields. This is sufficient to deflectmost energetic ions to curved paths, such that they miss the collectormirror. The high magnetic filed will lead to a slight elongation of theplasma source volume, but this can be tolerated. The coil has to bemounted on some support structure. It is conceivable, to mount the coilinside or outside of the vacuum chamber.

[0141] The radius of the curvature of a charged particle in a magneticfield is governed by the equation of motion:

F=q(v×B)

[0142] From which we can derive that the magnetic rigidity (B*R) for anion of mass M, accelerated to a voltage V is given by:

B*R=144(M*V)^(0.5)

[0143] Applying this case where we want to deflect a singly charge Xeion (mass 132) accelerated to 1000 Volts we get a rigidity of:

B*R=144(132*1000)^(b 0.5) (G-cm)=52,318 G-cm

[0144] Therefore if we want the ion to move in a circular orbit ofradius 10 cm we require a magnetic field of 52,318 G-cm/10 cm which isequal to ˜5232 gauss.

[0145] In general to deflect ions of different masses and energies wemay require stronger or weaker fields. The configuration of the magneticfield can also be adjusted to optimize the shielding power for the EUVoptics by winding coils in various configurations or using combinationsof coils and permanent magnets to achieve the desired field profiles.For these fields a coil can be placed either outside the vacuum vesselor interior to it. The current driving a coil required to produce agiven magnetic field can be easily calculated.

Honeycomb Debris Shield

[0146]FIGS. 9A, 9B, and 9C show examples of a special preferredembodiment utilizing a tapered powder-formed cellular honeycomb body asthe debris collector with an ellipsoidal radiation collector. The debriscollector is preferably produced utilizing one of the techniquesdescribed in U.S. Pat. No. 6,299,958 which is incorporated by referenceherein. The debris shield is produced through a reforming procedurewherein a precursor honeycomb, shaped from a plasticized powder batchmaterial, is filled with a compatible plastic filler material and thenshaped by forcing the filled honeycomb through a conical shaped form.The process forces a shrinkage of both the filler material and thehoneycomb structure. The structure now conical shaped is then removedfrom the form and the filler material is removed by a process such asmelting it. Then the now conical-shaped honeycomb is then hardened suchas by sintering. FIG. 9A is a three-dimensional cutaway drawing showingpinch region 100, honeycomb debris shield 102 and a portion ofellipsoidal shaped radiation collector-director 104. FIG. 9B shows across-section view of the FIG. 9A components along with ray traces 106A,B, C and D of four of the rays from pinch region 100. FIG. 9C shows howadditional ellipsoidal elements can be nested to focus more of thelight. Preferably 9 or 10 elements are nested within the outsideellipsoidal element. The powders, binder material and filler materialcan be chosen from the ones listed in the U.S. Pat. No. 6,299,958. Thechoice of material should be made recognizing the need of the debrisshield to withstand intense extreme ultraviolet radiation. A preferredchoice is powder and other material selected to produce cordieritecomprised of silicon manganese and aluminum.

Active Materials and Buffer Gas Choice of Active Materials and BufferGases

[0147] Several active materials and buffer gases are available forgenerating EUV light in the wavelength range of about 13.2 nm to 13.8nm. Preferred active materials are xenon, tin or lithium. These threeactive materials are discussed above in the section entitled, “Sourcesfor 12-14 nm EUV”. Indium, cadium and silver are also possiblecandidates. If one of the above materials are used as the activematerial than a noble gas such as helium neon or argon should be used asthe buffer gas. Nitrogen and hydrogen could be added to the potentiallist of buffer gases especially if xenon is the active material. Theactive materials which are metals are in most embodiments added to thedischarge chamber as vapors although they could be added as liquids orsolids and may be added in the form of a solution or powder.

[0148] All of these active materials are chosen because they provide anemission line in the desired range of 13.2 to 13.8 nm and as explainedabove, this is because reflective optics are available with relativelygood properties for UV light in this range. If and when good opticalcomponents become available in other wavelength ranges lower or higherthan this range, then the periodic table and corresponding emission lineliterature should be searched for alternative active materials. Also,buffer gases are not limited to the ones set forth above.

Injection Through Anode

[0149]FIG. 18A shows features of a preferred embodiment of the presentinvention in which the active gas in this case Xe (mixed 1 part and 14parts with helium) is injected through the anode. The buffer gas (inthis case 100% He) is injected at 12 in the region downstream ofcollector-director 8. Debris collector 6 comprises nested conicalsections providing narrow passageways in line with rays extending fromthe center of the pinch region to collector-director 8. Thesepassageways permit about 85% of the photons directed towardcollector-director 8 to pass but retards substantially the passage ofdebris generated in the pinch region which follows paths much morerandom than the EUV light. Gas is exhausted from vacuum chamber 10through port 14 by a 40 liter per second vacuum pump. Therefore, buffergas flow from gas feed 12 through the narrow passageways in debriscollector 6 further retards the passage of debris from the pinch andalso retards flow of the Xe active gas from the pinch region into theregion of chamber 10. Therefore, substantially all of the debris fromthe pinch region and active gas injected through port 24 is eitherexhausted through port 14 or coats the surfaces of the debris collectoror the inside walls of the vessel upstream of the debris collector. Thisavoids contamination of collector-director 8 by debris from the pinchand minimize attenuation of the beam by xenon gas since the flow ofbuffer gas through the narrow passageway in debris collector 6 preventsany significant quantity of xenon from entering the region downstream ofdebris collector 6.

Two Direction Gas Flow

[0150]FIG. 18B shows features of an embodiment of the present inventionin which two directional gas flow is utilized to permit a controlledconcentration of active gas near the pinch region with minimumconcentration of active gas in the downstream portion of the EUV beampath. In this case the active gas is introduced through the center ofanode 18A as shown at 24 FIG. 18B. In this preferred embodiment, theintroduced gas is a 1/15 to 14/15 mixture of xenon and helium. Helium isalso introduced at 12 as in the above embodiment. The introduced gasfrom both sources is exhausted at 14 with a vacuum pump of the typedescribed above. Gas flows are controlled to produce a pressure of about0.75 torr in the pinch region and a pressure of about 1 torr in thecollector-director region so that gas flow from the collector directorregion is much greater than the flow from the pinch region.

Upstream Injection of Active Gas

[0151]FIG. 18C shows another preferred technique for controlling debrisand the active gas and minimizing EUV absorption by the active gas. Gaspressure in the pinch region is about 0.5 torr. In this embodiment, gasflows within vacuum chamber 10 are arranged to help deter debris fromthe pinch region from reaching the region of collector director unit 8and to minimize the quantity of active gas in the region beyond theimmediate volume surrounding the pinch region. The active gas whichcould be, for example, xenon is injected about 3 centimeters upstream ofthe pinch region through nozzle 2 at a rate of about 5 SCCM and almostall of it is exhausted via a exhaust port 3 running through electrode18A along its axis at a pumping speed of 50 liter/second. The exhaustflow is provided by a vacuum pump such as design blower backed by anAnect Iwata ISP-500 scroll pump available from Synergy Vacuum a Canadiancompany. This provides a pump speed of 40 liters per second. The xenonis fed into nozzle 2 through gas pipe 4 running through the centralregion of debris catcher 6. Debris catcher 6 is comprised of nestedconical sections at 6A having surfaces aligned with light rays extendingout from the center of the pinch site and directed toward collectordirector 8. These nested conical sections provide a relativelyunobstructed passageway for EUV photons produced in the pinch which aredirected toward collector director 8. The passageways are narrow andabout 10 cm long.

[0152] Debris collector 6 collects (by condensation) tungsten vaporizedfrom tungsten electrode 18A. (If the active gas is lithium vapor, thevapor will also condense on the surfaces of debris collector 6.)

[0153] Buffert gas which in this embodiment is helium is injecteddownstream of collector director 8 as shown at 12 and most of the buffergas is exhausted from vacuum chamber 10 through exhaust port 14 by avacuum pump (not shown) of the type described above. About 90 percent ofthe helium flow passes through collector director 8 in the directiontoward the pinch region and all of the buffer gas passes through thenested conical section region 6A. As in the above example, this gas flowhelps deter debris produced in the pinch region from reachingdirector-collector 8 and also minimizes the amount of active gas in thepath of the light being collected and directed by collector-director 8to produce the output EUV beam. These features are important because anydebris accumulation on the surfaces of debris collector 8 reduces itsreflectivity and active gas in the EUV beam path will attenuate thebeam. Gas exhausted through port 3 is preferably filtered and exhaustedto the atmosphere. Gas exhausted through port 14 may also be exhaustedto the atmosphere without excessive gas cost since total helium gas flowin this system is only about 16 grams per hour. Alternatively, thehelium and/or the active gas may be separated and recirculated.

Lithium as Active Gas

[0154] Lithium vapor may more efficiently convert the pinch energy intouseful light at the desired wavelength range. Lithium is a solid at roomtemperature and a liquid between the temperature of 180° C. and 1342° C.Many methods are available to introduce lithium vapor into the dischargeand pinch regions. Lithium can be heated to its vapor temperature andintroduced as a vapor. It could be introduced as a solid or liquid andvaporized by the discharge or the pinch or it could be vaporized withother forms of energy such as a high power laser pulse or by some otherform of heating such as a resistance heating element, an electricdischarge or rf heating. Lithium can also be introduced as a compoundsuch as Li₂O, LiH, LiOH, LiCl, Li₂CO₃, LiF, CH₃ or their solutions inwater or other liquid.

[0155] Lithium may also be delivered to the pinch region by means oflaser induced evaporation or ablation. Lithium metal target 30 will beattached to a holder mounted from the central disk in the debriscollector as shown in FIG. 18D. In one preferred example, a KrF excimerlaser 32 produces a pulsed laser beam of 248 nm wavelength and energy of100 mJ to 200 mJ per pulse, with effective pulse length of 50 ns ispassed through a window 34 mounted on the upstream side of the anode.The light will pass through the hollow anode and be focused by means ofa lens 36 mounted external to the vacuum chamber to a spot ofapproximately 1 mm in diameter. This laser intensity and spot size issufficient to heat the Li metal at such a high rate that the temperaturerise is dominated by the latent heat of vaporization. The thresholdpower density required is about 5×10⁷ W/cm². At lower power Li can alsobe evaporated at a rate governed by its vapor pressure at a giventemperature.

[0156] In an alternative embodiment the central region of the centralelectrode as shown in FIG. 18A is packed with Li metal as shown at 38 inFIG. 17 and the laser beam is passed through the center of the debrisshield 8 as shown at 40 in FIG. 17.

[0157] In another technique by which we can deliver Li to the pinchregion is to attach the Li metal to a tungsten plate which is in turnmounted on a housing containing a permanent magnet. This arrangement ismounted on an insulating shaft from the debris collector. Li metal isfurther covered with a tungsten mask to expose only a small region ofLi. A radio frequency produced plasma is generated in the region infront of the Li target by means of an RF generator operating at afrequency of 500 MHz to 2.45 GHz. The discharge may be operated ineither pulsed or CW mode. In pulsed mode, the discharge will besynchronized with the plasma pinch. An RF power of 5000 W is generallysufficient.

[0158] The generated plasma will be composed of the buffer gas,generally He. He ions will be extracted from the plasma by applicationof a negative bias voltage onto the Li target. A bias of 500 V to 2000 Vwill be sufficient. He+ ions striking the Li will sputter Li atoms fromthe surface. Sputter yields over the bias energies mentioned vary fromapproximately 0.2 to 0.3 for normal incidence. Significantly higheryields can be expected for grazing incidence and for Li at elevatedtemperature.

Preionization Improvements

[0159] The DPF can be preionized with a variety of different techniqueseach of which have a beneficial effect on EUV output. The techniqueoriginally used in Cymer DPF is based on driving a set of spark plugtype pins 138 mounted in the outer electrode of the device as shown inFIG. 2A(2). These pins can be driven by a high voltage pulse such thethe RF simulator, or by the unipolar output of the 6000 seriescommutator. The voltage required to initiate breakdown using the RFsimulator or commutator is ±20 kV. Applicants have also demonstratedthat the preionization source can be located remote from the cathode butinside the main vacuum vessel. This is a coiled antenna. Applicants havealso successfully used a straight antenna for preionization.

[0160] This type of antenna can be either linear or shaped in the formof helical coil. The antenna can be driven either by an RF simulatordelivering high voltage (such as about) pulses at 13 MHz for 2 μs, thecommutator delivering either a positive or negative polarity pulse or byan RF amplifier. We have demonstrated to support (10 kHz pulserepetition rate). External preionization (antenna located outside of theanode/cathode region) has been shown to be the desireable mode ofpreionizing the negative polarity deep plasma focus. With positivepolarity DPF somewhat better preionization is achieved with the“internal” antenna shown in FIG. 1 above.

[0161]FIG. 32 shows that the timing of the preionization pulse must beadjusted relative to the DPF main pulse to achieve optimum effect. Ifthe preionization is too early (as shown at 92) or too late (as shown at93) the efficiency of the deep plasma focus is adversely affected.

Preionize Injected Gas

[0162] Applicants have discovered that gases in metastable states areeasier to preionize than stable gas. Gases can be put in metastablestates by ionizing them prior to injection into the discharge chamber.For example, FIGS. 2A(4), and 18A-E show gas injection techniques. Ineach case the injected gas could be placed in a metastable state by ahigh voltage discharge (such as with 15 kV pulses with durations of afew ns) or by RF preionization. These metastable states last about 50milliseconds so with a gas flow of about 1 m/sec there will be plenty ofmetastable atoms if the ionizing discharge is about 5 cm upstream of theorigin of the pinch discharge.

[0163] Another technique useful when xenon is the active gas is toinstall an RF coil around the xenon inlet to the discharge region.Applicants propose an RF frequency of 2 MHz to 2.5 MHz which causes abreakdown of the xenon gas in the inlet pipe. Alternatively, a highvoltage pulsed discharge in the xenon inlet pipe could be used. In apreferred embodiment a magnetic field is applied to direct xenon ions sogenerated to specific locations the pinch discharge is initiated.

Nozzle Assisted Preionization

[0164] The pressure for best production of EUV light in Applicantsfourth generation devices is in the range of about 100 mTorr or less.This pressure the discharge puts us on the left side of the Paschenbreakdown curve so that very high voltages are required for breakdown toproduce ionization. Ionization is much easier at higher pressures. Asolution, consistent with the techniques described in the previoussection, is to produce the preionization in the nozzle used to injecteither the buffer or active gas into the discharge chamber. Techniquesfor producing ions in the inject pipe are discussed above. Anothertechnique is to direct ionizing radiation to the injection nozzle frominside the chamber as shown in FIG. 31. This radiation is preferablydischarge produced UV light or x-radiation.

Hydrogen as Buffer Gas

[0165] Applicants have discovered that EUV optics in its prototypedevices become contaminated with carbon deposition. A 1 nm layer ofcarbon can cause a 1% relative reflective loss on multi-layer optics andmore (up to about 10% for grazing incident optics). One known techniqueis to add oxygen to the buffer gas to react with the carbon to produceCO and CO₂. However, oxygen can also react with the optics producingoxide which degrades the optics.

[0166] Applicants propose to add hydrogen to the buffer gas preferablyabout 20% to 50%. The hydrogen does not absorb at 13.5 nm, it etchescarbon and it also reacts with oxygen. Also, the hydrogen could addedonly periodically for short time periods as a part of maintenanceprogram to clean the optics and removed after the optics are cleaned.

Optimization Techniques Optimizing Capacitance

[0167] Applicants have discovered that the highest plasma temperatureexists when the plasma pinch event occurs simultaneous with the peak ofthe current flow from the drive capacitor bank. For a given anodeconfiguration and buffer gas density, the plasma front will travel downthe length of the anode in a given amount of time for a given amount ofcharge voltage. Maximum emission efficiency is obtained by adjusting thecapacitance value and charge voltage such that the peak capacitorcurrent exists during the plasma pinch event.

[0168] If a higher input energy level is desired and thus a highercharge voltage, then the drive capacitance must be reduced so that thetiming of the drive waveshape matches the plasma run down time along thelength of the anode. Since energy stored on a capacitor scales as thesquare of voltage and linearly with capacitance, the stored energy willincrease linearly with voltage as one decreases capacitance proportionalwith increases in voltage.

[0169]FIG. 13 is a drawing showing the measured drive capacitancevoltage, the measured anode voltage and the EUV intensity versus timefor a preferred embodiment with the capacitance properly chosen toproduce maximum capacitor current during the pinch. In this case, for a2 cm long anode, a He buffer gas pressure of 2.5 Torr and a C₁capacitance of 3 μF.

Optimum Shape of Central Electrode

[0170] Applicants have discovered with hollow anode configurations, thatthe plasma pinch grows rapidly along the axis once the pinch has beenformed, and will extend down the opening in the hollow anode. As thispinch grows in length, it eventually drops too much voltage along itslength and an arc-over occurs across the surface of the anode. Onesolution to prevent this arc-over makes use of a blast shield to providea physical barrier to the growth of the pinch length extending away fromthe anode as described above. Another solution, to reduce the rate ofpinch length growth down into the hollow anode, is to increase the opendiameter inside the anode narrow region as shown in FIGS. 14C and14D(1). This slows the growth of the pinch length and prevent arc-over.All previous literature shows a hollow anode with a constant dimensionhollow portion. FIGS. 14A, 14B, 14C and 14D show examples of pinchshapes for various hollow anode shapes. The configuration shown in FIG.14D shows the shortest pinch shape.

Exposed Length of Central Electrode

[0171] Since the plasma run down time determines where on the drivevoltage waveshape the pinch occurs, Applicants have been able to adjustthe duration of the pinch portion of the plasma focus device by changingthe amount of exposed anode and thus the duration of the rundown. Thebuffer gas density is dictated by a desired plasma pinch diameter, andthe drive capacitance is in practice limited to within a certain range.These two parameters, combined with the drive voltage determine thedesired run down time. The run down time can then be adjusted byincreasing or decreasing the length of exposed anode. Preferably, therun down time is chosen such that the plasma pinch event occurs duringthe peak in the drive current waveshape. If a longer plasma pinchduration is desired then the exposed length of the anode can be reduced,thus shortening the run down time and causing the plasma pinch to occurearlier in the drive waveshape.

RF Powered Vapor Production

[0172] Metal vapor delivery schemes described above depend on raisingthe anode temperature sufficiently high that the vapor pressure of metalreached a desired level. Such temperatures are in the range of1000-1300° C. for lithium and 2,260° C. for tin.

[0173] An alternative is to fabricate an RF antenna from a material suchas porous Tungsten infiltrated with Lithium. This porous Lithium filledTungsten antenna 50 is placed down inside the anode as shown in FIG. 15.RF power source 52 creates a plasma-layer on and near the antenna willdrive off atoms that are swept up by the gas flow 54 through the centerof the hollow anode and the Lithium atoms carried to the end of theanode. The rate of metal ion production is easily controlled by thepower level of the RF source. In addition, the porous Tungsten anode canbe maintained with this RF drive at a temperature sufficient for liquidmetal to wick up from a reservoir 56 placed at the bottom of the anode.

Electrode Cooling Cooling of Central Electrode

[0174] In preferred embodiments of the present invention the centralanode has an outside diameter in the range of about 0.5 cm to 1.25 cm.The central electrode absorbs substantial energy due to the plasma fallduring discharge and due to absorption of radiation from the plasmapinch. Cooling in the range of about 15 kw or more may be required.Because the gas pressure are very low there cannot be much cooling dueto convection through the buffer gas. Radiation cooling could only beeffective at very high anode temperatures. Conduction down the anodelength would require a very large temperature drop.

Heat Pipe

[0175] If lithium vapor is used as an active gas and is injected throughthe center of the anode the anode temperature may need to be maintainedat temperatures in the range of 1,000° C. to 1,300° C. or higher. Thishigh temperature of operation, substantial heat removal requirement,envelope considerations and the high voltage limit the choices ofcooling technique. One technology, however, a lithium (or other alkalimetal) heat pipe, offers the potential for a relatively simple androbust solution. Lithium heat pipes begin to operate efficiently attemperatures about 1000° C. The specific design of such devicestypically use refractory metals, molybdenum and tungsten, for the casingand internal wick and can therefore operate at very high temperatures.

[0176] The simplest embodiment would take the form of a tubular orannular heat pipe that is integral with the anode of the DPF for bestthermal coupling. A likely embodiment would be annular to enable thedelivery of liquid or vaporized lithium to the plasma of the DPF. By wayof an example, an 0.5 inch diameter solid heat pipe removing 15 kW wouldhave a watt density of 75 kW/in (11.8 kW/cm²). An annular heat pipehaving an OD of 1.0 inch and an ID of 0.5 inch removing 15 kW of heatwould have a watt density of 25.4 kW/in² (3.9 kW/cm²). Both of theseexamples illustrate the potential of this technology since wattdensities far in excess of 15 kW/cm² have been demonstrated with lithiumheat pipes. In operation, heat pipes have only a very small temperaturegradient along their length and can be considered as having constanttemperature with length for practical purposes. Therefore, the “cold”(condenser) end of the heat pipe will also be at some temperature at orabove 1000° C. To remove heat from the condenser end of the heat pipe apreferred embodiment may utilize radiative cooling to a liquid coolant(such as water) jacket. Radiative heat transfer scales as the fourthpower of temperature, therefore, high rates of heat transfer will bepossible at the proposed operating temperatures. The heat pipe would besurrounded by an annular water heat exchanger capable of steady stateoperation at 15 kW. Other embodiments may insulate the condenser end ofthe heat pipe with another material such as stainless steel and cool theouter surface of that material with a liquid coolant. Whatever techniqueis used, it is important that the heat pipe is not “shocked” with acoolant at the condenser, i.e., forced to be much cooler than theevaporator end. This can seriously impact performance. Also if the heatpipe temperature falls below the freezing temperature of the workingfluid at any point along its length (˜180° C. for lithium) it will notwork at all.

[0177] Restrictions to the operating temperature of components near thebase of the central electrode (anode) may require that heat transferredto this region be minimized. This condition may be accomplished, forexample, by coating the exterior of the heat pipe with a low emissivitymaterial near the region of lower temperature tolerance. A vacuum gapcan then be fabricated between the heat pipe and the desired lowertemperature components. Since vacuum has very low thermal conductivityand the heat pipe is coated with a low emissivity material, minimal heattransfer will occur between the heat pipe and the cooler components.Maintaining a controlled anode temperature under varying power loadlevels is another consideration. This may be accomplished by placing acylinder between the heat pipe and the water cooled outer jacket. Thiscylinder would be coated or finished for high reflectivity on its innerdiameter and for low emissivity on its outer diameter. If the cylinderis fully inserted between the radiating heat pipe and the water coolingjacket, radiation will be reflected back toward the heat pipe thusreducing the power flow from heat pipe to jacket. As the “restrictor”cylinder is extracted a greater proportion of the heat pipe's condensercan radiate directly onto the water jacket heat exchanger. Adjustment ofthe “restrictor” position thus controls the power flow which sets thesteady state operating temperature of the heat pipe, and ultimately theanode.

[0178] A preferred embodiment using heat pipe cooling is shown in FIG.16 shown in the drawing are anode 8A, cathode 8B, and insulator element9. In this case, lithium vapor is used as the active gas and isdelivered into the discharge chamber through the center of anode 8A asshown at 440. Anode 8A is cooled with lithium heat pipe system 442comprising lithium heat pipe 444. Lithium within the heat transferregion 446 of heat pipe 444 vaporizer near the hot end of the electrode8A and the vapor flows toward the cooler end of the heat pipe where heatis transferred from the heat pipe by radiative cooling to a heat sinkunit 446 having a heat sink surface 448 cooled by water coil 450. Thecooling of the lithium vapor causes a change in its state to liquid andthe liquid is wicked back to the hot end in accordance with well knownheat pipe technology. In the embodiment a restrictor cylinder 452 slidesup and down as shown at 454 inside heat sink surface 448 based on adrive which is part of a temperature feedback control unit not shown.The anode heat pipe unit also preferably comprises an auxiliary heatingsystem for maintaining the lithium at temperatures in excess of itsfreezing point when the plasma pinch device is not producing sufficientheat.

Water Cooling of Central Electrode

[0179] Another preferred method of cooling the central electrode isshown in FIGS. 20, 20A, 21 and 22. In this case water under pressure iscirculated through the central electrode. Central electrode 8A as shownin FIG. 20C is comprised of two parts, a discharge portion 8A1 comprisedof single crystal tungsten (available from Mateck GMBH, Fuelich, Germanyand lower part 8A comprised of sintered tungsten. The outer electrode 8Bis made in two parts, a lid 8B1 and a base 8B2, both comprised of anoxide hardened copper material sold under the tradename Glidcop. Theoxide material is alumina. The outer electrode is made in two parts toprovide water passages 460 for cooling the outer electrode. Theelectrodes are insulated from each other by main insulator 462 comprisedof boron nitride or silicon carbide, a layer 464 of alumina deposited onstainless steel base 8A3 and a polymide 466 (preferably Kapton asavailable from Dupont). The water path through the central electrode isshown by arrows 468 in FIG. 20C. Cylindrically shaped stainless steelpartition 470 separate the supply and return flow in the electrodes.Parts 8A1, 8A2 and 8A3 are braised together using a gold/nickel orgold/copper braze material such as Niord or 50 An-50c.

Plasma Pinch with Radial Run-Down

[0180] Preferred embodiments of the present invention utilizes the pulsepower features, the radiation collection features and the debris controlfeatures described above with any of the electrode arrangement asdescribed in FIGS. 2A, 2B, 2C and 2D. This electrode arrangementprovides advantages and disadvantages as compared to electrodeconfiguration such as that shown in FIG. 21. The electrodes have greatersurface area so that thermal problems may be minimized. There also couldbe less filamentations of the discharge and perhaps better plasmaconfinement and possibly better radial stability. Applicants believethey can design the electrodes to produce pinches along the axis of theelectrodes as shown in FIG. 21.

Use of Multiple EUV Sources

[0181] As indicated above a preferred application of the presentinvention in for lithography light sources for future machines, at leastthe production versions, have not yet been designed and built. It ispossible that illumination power may exceed the illumination power thatcan be conveniently produced by a single EUV source source utilizing thetechnology described herein. In this case two or more EUV sources couldbe combined to provide the illumination needed. Preferably the lightfrom each of the sources would be collected using techniques similar tothose described herein and projected on a single slit which would be thesource for the lithography equipment.

Integration with Litho Machine

[0182] In preferred embodiments portions of the EUV light source unit isintegrated directly into a lithography unit such as a stepper machine asshown in FIG. 2A(21). The integrated parts may include the commentatorand the compression head of the solid state pulse power unit and thevacuum vessel which includes the electrode set, debris shield andradiation collectors and turbo-molecular vacuum pumps all as shown at120 in FIG. 2A(21). Support equipment (including electronic controls,high voltage power supply, resonant charger, power distribution systemand fluid management for cooling water and gas control) are located in asupport equipment cabinet separate from the lithography unit (whichcould be in a separate room if desired) all as shown at 122. Roughvacuum pumps and high pressure water pumps are located in a thirdcabinet 124 which also could be in the separate room, in lithographyunit 126 are illumination optics, reticle, reduction optics and waferhandling equipment.

Electrode Erosion Minimizing Erosion

[0183] Applicants' experiments with their early prototype EUV deviceshow that electrode erosion is a serious issue and Applicants havedeveloped several techniques for dealing with this issue. Applicantshave discovered through experiments with their fourth generation plasmapinch device that the inductance in the discharge circuit increasesdramatically at the time the pinch occurs greatly reducing the currentflow and producing an increasing electric field between the electrodes.As a consequence a second breakdown occurs between the anode and thecathode generally near the tip of the anode as shown in FIG. 2A(2). Thisproduces erosion at the location of the breakdown. Applicants propose tominimize this problem by providing a means for promoting their postpinch discharge at a location where erosion is not a problem. Onetechnique to inject a plasma containing gas in lower region between theelectrodes to produce the post pinch in this lower location far awayfrom the anode tip.

Sputter Replacement of Material Eroded from Anode

[0184] Applicants' experiments with its fourth generation device haveshown substantial anode erosion with long-term operation. As indicatedabove the principal expected use of these plasma pinch devices is forintegrated circuit production. This means the device must operatesubstantially continuously for many days or weeks between maintenancedown times. Therefore, techniques must be found for increasing electrodelifetimes. A potential technique is to provide a sputter source forsputtering electrode material onto one or both of the electrodes. FIG.25 is a sketch showing two-tungsten sputter sources for providingsputtered tungsten to replace electrode erosion. Applicants discoveredthat short pulse high voltage driven electrodes used for preionizationwas producing sputter ions which collected on the sides of the anode andon the cathode. The side of the anode is also the location of most ofthe electrode erosion. Therefore, Applicants propose to providesacrificial electrodes of the same material as the anode and cathodespecifically designed to erode by sputtering. These sacrificialelectrodes will be positioned so that sputtered electrode material isdirected to regions of the anode and/or cathode suffering worse erosion.Preferably the sacrificial electrodes are designed so that they can beeasily replaced or periodically extended into the discharge chamber asthe erode. Some of the sputtered material will collect on insulatorsurfaces, but Applicants have leaned that sputtered tungsten depositedon insulator surfaces in these devices is not a problem.

Insulator Covered Electrodes

[0185] Applicants have discovered in actual experiments that centerelectrode erosion can be greatly reduced by covering the side wall ofthe center electrode with insulator material. By covering with insulatormaterial portions of the electrode which would otherwise face highcurrent densities, the post pinch discharge current is forced to spreadout over a larger area in a different region of the electrode. Thistechnique can be employed to reduce the current density in the area ofelectron or ion impact on the anode or cathode, respectively. Thereduced erosion rate leads to reduced debris generation and longerelectrode lifetime. There is still some erosion and debris from thesliding discharge across the insulator, but it is not so severe as theelectrode erosion. The so-called “flash-over arcing” which leads to higherosion rates occurs only on conductive surfaces. It can therefore beeliminated in regions where the electrode is covered by the insulator.

[0186] Thus, a preferred embodiment is a dense plasma focus with theusual anode and cathode configuration, but without a sliding dischargealong the outer diameter of the inner electrode (run-down length).Instead, the inner electrode is covered by a long insulator tube whichprotrudes, i.e., the diameter of the inner electrode is eliminated. Eventhought the effective inductance is slightly increased, an intense pinchstill occurs on the axis leading to EUV generation. In contract toconventional dense plasma focus devices, there is no run-down occurringalong the inner electrode. The inner surface of the inner electrode mayalso be covered with insulator material to eliminate flash-over arcingin this region. This insulator has to have the appropriate innerdiameter in order not to reduce the pinch size and EUV output.

[0187] Preferred embodiments are in FIG. 26A and 26B. In FIG. 26Ainsulator 60 covers the outside surface and in the FIG. 26B embodimentinsulator 62 covers the inside surface in addition to insulator 60 onthe outside. The anode in both FIGS. is identified at 64 and the cathodeat 65.

Pyrolytic Graphite Electrodes

[0188] In a preferred embodiment the discharge surface of the anodeshown at 8A in FIG. 2A(2) is covered with pyrolytic graphite. The bodyof the anode is copper or tungsten. An important advantage of thisdesign is that carbon is 15 times lighter than tungsten (the principalprior art anode material). Therefore, the carbon debris is much easierto deal with in a debris shield. Also graphite does not melt; itevaporates. Preferably the graphite should be applied so that the atomicgraphite layers are aligned perpendicular to the surface to improvethermal conductivity and to minimize erosion. Prerably an interlayer isapplied between the pyrolytic graphite surface material and thesubstrate electrode material to minimize thermal stresses.

Electrode Replacement Shutter with Seal

[0189] When the plasma focus source components and collector arecontained in the same chamber any maintenance of the source requiringventing will have disadvantageous effects on the collector mirrors andalso on the debris trap. A separation of these components into twochambers with respect to vacuum should be very beneficial. However,prior art designs with respect to position of debris trap and collectionoptics just do not provide the space required to accommodate a gatevalve between the two chambers.

[0190] Applicants have developed techniques for venting the sourcechamber for maintenance (like electrode replacement) while keeping thecollector chamber under (near-) vacuum during this time. The sourcechamber 69 will require more frequent venting compared to the ventingrequired for the collector chamber 70. The collector mirrors 66 and alsothe debris trap 68 will be protected when maintenance is carried out onthe source by use of the proposed shutter. Therefore the lifetime of thecollector (and perhaps also of the debris trap) will be greatlyincreased. Since a very short distance is required between the pinchsource volume 71 and the debris trap and collection optics entrance inpresent designs, there is usually not enough space available toaccommodate a separating gate valve. When the proposed shutter with sealtowards the collector chamber is introduced, only very little space isrequired to accommodate it. The collector chamber can be kept under(near) vacuum, since the shutter will be pressed against the sealingsurface by the ambient pressure of the vented source chamber.

[0191] The advantage of the present design is illustrated in FIGS. 27Aand 27B. The prior art drawing FIG. 27 shows an arrangement with a gatevalve 72 separating the source and collector chambers. However, presentdesigns require a distance of 100 mm or less from the plasma sourcevolume to the entrance of the grazing incidence collector optics andthus usually do not provide enough room to accommodate a gate valve. UHVgate valves from vacuum suppliers like VAT with 8 inch (200 mm) or 10inch (250 mm) opening diameter have a flange-to-flange distances of 80to 100 mm. Therefore, such gate valves are omitted in present designs.This has the big disadvantage that each time when venting formaintenance of the source is required, the collector chamber is alsovented. Each venting cycle has disadvantageous effects for the verysensitive collector optics. Furthermore, the pump-down time is longerfor the collector chamber compared to the source chamber since itsvacuum requirements are more severe. If the collector chamber does notneed to be vented each time when the source chamber is vented, severaladvantages exist: The collector optics contamination is reduced and theoptics lifetime is increased. The system maintenance down-time isdecreased because no pump-down of the collector chamber is required atthe end of the maintenance work. The sensitive debris trap is alsoprotected better.

[0192]FIG. 27B shows a proposed mechanical shutter 74 with vacuum sealfrom the source to the collector chamber. The shutter has an o-ring sealon the side facing the collector chamber just like the plate of a gatevalve. The space required to accommodate this shutter is only 20 mm orperhaps even only 10 mm. In contrast to a gate valve the shutter canprovide a vacuum seal only with respect to collector chamber and not forthe source chamber. However, this is sufficient, since in most casesonly the source chamber needs to be vented (shutter in closed positionas shown in the figure). When the collector chamber needs to be vented,the source chamber can always be vented, as well, without anydisadvantages (shutter in open position).

[0193] When the shutter is approaching the closed position, it ispressed with its o-ring seal against the sealing surface of thecollector chamber by a notch or protrusion near the shutter endposition. The sealing surface may be conveniently located on the outercircumference of the debris trap (holder), for instance. At the start ofthe source chamber venting, the increased pressure in the source chamberwill push the shutter further against its sealing surface with a forcewhich will increase with the increase of the pressure in the sourcechamber. At the beginning of the venting some small leaks may stillexist towards the collector chamber, but this can be tolerated. When thesource chamber is at high (atmospheric) pressure, the force pushing theshutter against its sealing surface will be so large due to therelatively large shutter area that a high-vacuum seal is established.This is sufficient to protect the collector optics (and debris trap). A(minor) disadvantage is that the sealing shutter has to be integratedinto the collector (or source) chamber design (preferentially right nextto the connecting vacuum flange). But the major advantage is that thespace required for the extra 2 flanges of the gate valve and some of itswidth can be avoided. Therefore, such a shutter can be accommodated evenwhen the required separation from the source to the debristrap/collector entrance is very small.

Replaceable Electrode Module

[0194] Another technique to simplify electrode replacement is to designthe EUV device for replacement of the electrode, the debris collectorand the first collector as a single module. For example, referring toFIG. 19, collector 42 would a port of a module comprised of anode,cathode and debris collector and collector 42. The system would permitthese components to be replaced as a unit in a minimum period of time toreduce maintenance down-time. This results in quick replacement of theelectrodes which degrade because of erosion and the debris collector andfirst collector optics which degrade because of contamination witheroded material.

Example of an Optimized Dense Plasma Focus Device Optimization Efforts

[0195] Applicants have devoted considerable effort to optimizeperformance of their fourth generation dense plasma focus device shownin cross section in FIG. 2A(1) for efficient generation of EUVradiation. A side view of the system with vacuum chamber is shown inFIG. 2A(3). Performance parameters included in their investigations areHe and Xe pressure and flow rates, electrode geometries, pre-ionizationcharacteristics, and duty factor related performance issues. In theseinvestigations Applicants found that the location of the He (buffer gas)and Xe (working gas) gas injection ports as well as the pressures andflow rates of the gas mixture components had a strong impact on EUVemission efficiency. Additional constraints on the gas recipe are alsoderived from gas absorption of the EUV radiation and the desire toprovide debris mitigation properties. Best results to date have beenobtained with an axially symmetric buffer gas injection scheme coupledwith axial Xe injection through the central electrode. The highestconversion efficiency obtained was 0.42% at 12.4 J of input energy.Measurements of energy stability show a 10% standard deviation at nearoptimum EUV output. The matching of the drive circuit to the pinch asdetermined by the damping of the voltage overshoot waveforms was foundto depend strongly on the He and Xe pressures. Energy Dispersive X-Ray(EDX) analysis of the debris emitted from the source shows that theprimary sources of the debris are the central electrode and theinsulator. No evidence of cathode material has been found. In additionto efforts toward more efficient operation, first phase efforts ofthermal engineering have been undertaken, which have led to continuousoperation at 200 Hertz with conventional direct water-cooling. Thesystem can be operated at higher repetition rates with proportionallylower duty cycles. The data shows the distribution of thermal powerthroughout the whole system. This more detailed understanding of thethermal power flow allows Applicants to better determine the ultimatehigh volume manufacturing potential of this source technology.

[0196] Applicants have demonstrated significant gains in performancewith conversion efficiencies approaching those of the more mature laserproduced plasma sources. The particular specifications which the lightsources must meet are tightly coupled with the design of the entireillumination system. Key source parameters which must be measured are:operating wavelength, in-band EUV power, out-of-band power, source size;maximum collectible angle, high repetition rate scaling; pulse to pulserepeatability and debris generation from plasma facing components.

[0197] Applicants' early efforts in DPF development were directed atdeveloping the basic pulsed power technology required to drive a sourceof this type. High conversion efficiency was demonstrated with Li vaporas the active radiating element at high stored energy (25 J). Thesestored energies were too high for practical scaling to high repetitionrate operation. Development of the 4th generation machine allowedApplicants to use Xe as the active species. Their recent efforts havebeen focused on optimizing the performance of the DPF with Xe as thesource gas. To facilitate this effort they have investigated pulsedpower development, plasma initiation and characterization, EUVmetrology, debris mitigation and characterization, thermal engineering,and collector optics development.

System Description

[0198] The fourth generation of Dense Plasma Focus system developed byApplicants utilizes a power system with solid-state switching andseveral stages of magnetic pulse compression (as shown in FIG. 1 anddescribed above) similar to that used in Cymer's excimer lasers, inorder to generate the high voltage, high peak power pulse required bythe DPF to generate EUV light. These systems begin with a chargingvoltage of 1300 V and generate an output pulse applied to the DPF of ˜4kV with a risetime of less than 50 ns. Although current measurementshave not yet been directly performed, circuit simulations based on thevoltage waveforms from typical experiment operation predict that theoutput DPF drive current peaks at a value of ˜50 kA, with a dI/dt of 675kA/∝s. It is this combination of high peak current and high dI/dt thatallow the DPF to function efficiently.

[0199] The most important features of this fourth generation device isdescribed in FIG. 33 along with a bullet list of the advantages of thedeep plasma focus device. As explained elsewhere Applicants havedemonstrated conversion efficiencies (the ration of: in-band EUVradiation at an intermediate focus to electric power input) of about0.5%. As of the filing of this application, Appliants have demonstratedthe following system performance parameters:

Current Source Performance

[0200] EUV efficiency with Xe, (2% BW, 2π sr) >0.45% EUV energy perpulse (2% BW, 2π sr) ˜55 mJ Average source size (FWHM) ˜0.4 × 2.5 mmSource position stability (entroid) <0.05 mm, rms Continuous repetitionrate 1000 Hz Burst repetition rate 4000 Hz Energy Stability ˜7%, rmsAvg. EUV Output Power (2% BW, 2π sr)  50 Watt EUV output Power, Burst(2% BW, 2π sr) 200 Watt

[0201] Collection efficiency is about 20 to 30 percent and about half ofthe collected EUV in band radiation can be delivered to the intermediatefocus utilizing the technology described herein. Thus, the demonstratedEUV power at the intermediate focus is currently about 5 Watts on acontinuous basis and 200 Watts in burst mode. With the improvementsdescribed herein Applicants expect to increase the continuous power atthe intermediate focus to at least 45.4 Watts within the near future andultimately to 105.8 Watts. Burst mode performance will be roughlyproportionately greater.

[0202] Six fourth generation DPF machines have been built and are beingused for a variety of experiments on system optimization,pre-ionization, power system development, debris mitigation, thermalmanagement, and collector design. For those experiments not requiringhigh repetition rates (˜1 kHz and above), charging power for thesemachines is simply provided by resistive charging from a set of DC powersupplies. Those DPF systems that do require high rep-rate capability arebeing charged with a resonant charging system which charges the initialenergy storage capacitor, C0, to a voltage of 1300 V in less than 250μs. These resonant charging systems also provide energy recovery,storing the energy which is not utilized by the DPF or dissipated inheat and using this recovered energy for the next pulse. This reducesthe amount of power required by the main power supply and also helpswith other issues such as thermal management.

Measurements

[0203] In this section Applicants present an overview of measurementsperformed on one of Applicants low-duty-factor sources operated at lessthan 50 Hz. They show the dependence of the EUV output and conversionefficiency on gas recipe, present data on the out of band emission, andshow measurements of the source size and position stability.

[0204] Over the past year significant progress has been made inunderstanding some of the empirical dependencies of the EUV output onelectrode geometry and gas dynamics issues. Significant changes in theapparatus, as compared with earlier generations include a new cathodedesign which allows gas to be injected symmetrically around the anoderegion, and a system for injecting He and Xe mixtures through the anodeelectrode. The gas delivery system was modified to allow combinations ofHe and Xe to be injected into different sections of the DPF system. Aschematic of this system is shown in FIG. 1. Gas control is performedvia two mass flow controllers and a high accuracy capacitance manometer.The system is operated in a constant-pressure mode. Xe is injected in aconstant-flow mode, and He makeup gas is added in order to reach thetarget operating pressure. In this mode the He flow rate depends on thepumping speed of the system. Dependence on gas flow rates wasinvestigated by testing different pumping configurations.

[0205] Radiation emitted from the pinch along the axis passes through anaperture into a differentially pumped diagnostic chamber 204. Gasabsorption in the measurement vessel is minimized by maintaining thepressure below 5 mTorr. For these measurements the diagnostic vesselentrance was located 5 cm from the pinch region. No correction for thegas attenuation in the main DPF vessel along the 5 cm path nor in thediagnostic vessel is performed. The radiation from the pinch isreflected from a Mo/Si multi-layer mirror and is directed through a 1 ∝mthick Be foil onto an un-coated IRD AXUV-100 photodiode. A typicalmeasurement sequence consists of recording the voltage waveforms on thepulsed power system, the DPF anode, and the photodiode as a function ofthe experimental parameters. Data acquisition and control of the gassystem are performed via a computer interface.

[0206] The representative dependence of the in-band EUV signal (at 13.5nm, into 2% bandwidth, into 2 πsr) on the Xe flow rate is shown in FIG.2A(5) at a constant operating pressure of 350 mTorr and at a fixedcharging voltage on the first stage capacitor of the pulsed powersystem.

[0207] A significant increase in the EUV output from the source wasobserved when He was injected around the anode and Xe through thecathode at 20 Hz source operation compared with He injection into themain DPF vessel. Additional improvement was observed by increasing theHe gas flow rate via the addition of pumping capacity. The effect ofhigher pumping speed is to make the EUV output less sensitive to the Xemass flow set point and to increase the measured EUV output.

[0208] Similar measurements were performed as a function of He pressureat a constant Xe flow rate and a voltage of 1300 V on the firstcapacitor stage C2 as shown in FIG. 1. FIG. 2A(6) shows the voltagewaveform on the final stage capacitor (C2) and the in-band 13.5 nmphotodiode signal for He injection around the anode. The EUV signalstrongly depends on the He pressure. Examination of the C2 waveformshows that the energy recovered by this capacitor due to underdampedresponse depends on the gas recipe. A similar dependence was observed asa function of Xe flow variation.

[0209] The energy dissipated in the pinch region is calculated from thedifference in stored energy on the C2 capacitor. At 1500 mT He pressure,approximately 70% of stored energy is dissipated in the pinch region(8.8 J), while at 200 mT, the corresponding value is 96% (11.9 J). Thisdependence is illustrated in FIG. 2A(7) where the photodiode signal,initially stored energy, recovered energy and dissipated energy areplotted as a function of the He pressure. The EUV signal increases byapproximately a factor of 10 over this range. A further decrease in theHe partial pressure results in a sharp drop in the EUV yield not shownin these data.

[0210] Another interesting feature of the dependence on the gas pressureis the shift in the onset of EUV emission as measured by the photodiode.At the constant Xe flow conditions used, this variation of the pressurefrom 180 mT to 1500 mT results in a shift of 150 ns of the EUV emission.From classical snowplow and slug models of the DPF operation Applicantsexpect the characteristic axial and radial transit times for the plasmashock front to scale with the square root of effective mass density.This scaling needs to be confirmed for this configuration, and theproportionality constant may be related to the effectiveness of theshock front in sweeping the mass out of the electrode region.Calculations of this effect, based on a one-dimensional snowplow modelsuggest that axial and radial effective masses may be significantly lessthan those derived from the actual gas pressure.

[0211] The dependence of the average in band EUV energy and energyefficiency on the dissipated energy at fixed gas flow conditions isshown in FIGS. 2A(8) and 2A(9), and 4 b. These data were taken with thesource conditions optimized at the peak EUV output. Lower energy inputwas obtained by reducing the charging voltage while leaving all otherparameters fixed. The data shown here are for the optimum conditions ofthe present experiment as well as for the configuration presented in [1]employing a different gas recipe and anode geometry. At 10 J a 70%increase in conversion efficiency (CE) is obtained, as compared with theprevious configuration. Although the energy coupled into the pinchdepends on the gas recipe, we can see that the dependence of EUV energyshown in FIGS. 2A(6) and 2A(7) is primarily due to variation in gas flowand not to the change in coupling.

[0212] Two types of measurements of the EUV radiation lying outside the2% bandwidth around 13.5 nm were performed. The experimental setup forthese measurements is shown in FIGS. 2A(8) and 2A(9). The first type ofmeasurement compared the total radiation from the pinch on axis with thefraction transmitted through a CaF2 window transmitting in the 130nm-1300 nm band. These results show that ˜0.5% of the total radiationemitted from the pinch lies in the CaF2 band between 130 nm and 1300 nm,and are similar to previous results obtained by Applicants. In thesecond measurement the fraction of radiation emitted from the pinchreflected from one Mo/Si multilayer (ML) mirror and detected by theAXUV-100 photodiode was compared with the fraction of radiationtransmitted through a 1 ∝m Be foil and reflected by the ML mirror. Thesignal, measured on the photodiode with no filter in place afterreflection from the ML mirror gives the sum of in-band and out-of-bandcomponents. Insertion of a Be filter limits the measurement to thein-band fraction only. Therefore by subtracting the in-band fraction ofradiation corrected for the ML mirror transmission from the total signalwith no filter we conclude that ˜15% of the total radiation reflected byone ML mirror is out of the 2% band around 13.5 nm.

[0213] Measurements of source size and centroid motion were performedwith the source tuned for peak output. A pinhole camera employing aback-illuminated CCD array and a Be filter was used. The source imagesare shown in FIG. 2A(10). These images were taken with the camerapositioned on axis. Measurements were also taken at an angle of 68degrees. The average source size (averaged over 100 pulses) wasdetermined to be 0.25 mm×2 mm full-width-at-half-maximum. Thepulse-to-pulse EUV source centroid displacement is plotted in FIG.2(A)11. The average displacement is approximately 50 ∝m.

[0214] Using the pinhole camera technique we can obtain an estimate ofthe EUV energy stability by integrating the intensity in each frame andcalculating the standard deviation of this quantity. The results show9.5% (1 ) intensity fluctuation. This measurement compares well withmeasurements of the energy stability performed with the standardmeasurements using a Be filter, ML mirror, and AXUV-100 photodiode.Additional experiments that will be performed with this diagnostic willinclude correlation of EUV source size with the in-band energy.

[0215] From the on-axis images we also conclude that there is no EUVproduction originated from an interaction of the pinch with the anodeend wall. The maximum EUV intensity is observed in the center of thepinch where Xe gas is injected through an aperture in the anode. No EUVemission is observed at the periphery of the pinch where it contacts theanode end wall.

High Repetition Operation

[0216] Stable operation of the source at high repetition rates isimportant for high exposure dose and accurate dose control. The burstmode operation of this fourth generation light source was improved.Using a resonant charging scheme with 10 J input energy (similar to thatemployed by Cymer's excimer lasers), the maximum burst emission periodwas increased to up to 300 pulses at repetition rates of 2 KHz.

[0217] The time-integrated in-band energy of the EUV pulses was measuredusing the multi-layer mirror—Be foil—photodiode detection schemedescribed above. The in-band energy vs. pulse number data are shown inFIG. 2A(12). When the repetition rate was increased from low to highrates with no changes of the gas mixture, a severe reduction of the EUVoutput energy was observed with increasing burst pulse number. By makingappropriate adjustments of the gas recipe it was possible to tune theoutput in order to obtain relatively stable EUV pulse energy for 300pulses long bursts at a 2 kHz repetition rate. As shown in the figure,after a transient period lasting for about 10-15 pulses the outputenergy stays at high values for the remainder of the burst. Thecorresponding measured standard deviation of the energy stability inthis mode is 10%. At the present level, we have not reached anyfundamental scaling limitations for high-repetition-rate operation and afurther performance increase should be possible with upgraded pulsedpower and thermal management schemes.

Debris Mitigation

[0218] Applicants have exposed Mo and Pd coated silicon wafers to thedebris produced by the DPF in an effort to evaluate the main source ofthe debris, and the debris deposition rate on the collector optics. Thesource configuration for these tests consisted of a tungsten anode,alumina insulator and brass cathode. Samples were exposed to 4.105pulses at 30 Hz, at a distance of 5 cm (Mo sample) and 11 cm (Pd sample)away from the pinch. The arrangement and placement dimensions are shownin FIG. 10. After exposure the samples were analyzed by EnergyDispersive X-Ray (EDX) analysis. The results, summarized in Table 1below, show that anode (W) and insulator (O, Al) materials were found atboth distances, 5 cm and 11 cm.

[0219] No sign of cathode material was observed. A small fraction of Xewas found on the Mo sample at 5 cm. This may be a signature of energeticXe ions produced by the DPF or simply Xe incorporated into the thin filmcoating. The presence of He could not be detected by EDX. The presenceof a weak but detectable Mo signal at 5 cm is an indication that thedeposited debris is between 0.5 ∝m and 2.0 ∝m thick, which is thetypical penetration depth for EDX analysis. This gives us an estimate ofthe debris generation rate at 1-4.10-3 nm per pulse on axis at 5 cm fromthe pinch.

[0220] A simple optical technique was tested to characterize thedeposition of debris generated by the DPF. The absorption of metals inthe visible region of the spectrum is generally high. The correspondingoptical thickness up to which appreciable transmittance occurs isgenerally well below a quarter wavelength in this region so thatinterference fringes are not observed. According to Lambert-Beer's law:

T=e^(−α*L)

[0221] where T is the transmittance, α is the absorption coefficient andL is the film thickness. Therefore the absorbance A, defined asLog₁₀(1/T), is proportional to the film thickness if α is independent ofL. If L is proportional to the number of pulses, then from a measurementof the absorbance of a coating on a transparent sample due to debrisproduced by the DPF as a function of the number of pulses the debrisdeposition rate per pulse may be determined. Experimental verificationof this proportionality is plotted in FIG. 11.

[0222] Measurements of the absorbance allow one to compare the debrisdeposition rate on witness samples under different DPF operatingconditions. We used this method as the primary means for obtaining theangular distribution of the debris, as well as for the debris reductionfactor due to the insertion of a debris shield.

[0223] To evaluate the effectiveness of the debris shield concept asimple single-channel test setup was designed and built. The geometryand critical dimensions are shown in FIG. 2A(15). Glass samples wereplaced at 6 cm from the pinch either facing the pinch directly or aftera series of metal cylinders with 1 mm diameter channels drilled throughthem. Tests were performed with 1 cm and 2 cm channel lengths. Duringthe tests total pressure in the chamber was 0.7 Torr with Heliuminjection into the main vessel and Xe was injected through the anode. Bycomparing the debris film thickness using the absorbance technique, forsamples which were exposed to the same number of pulses at the sameoperating conditions but with different debris shield lengths, we cancalculate a debris reduction factor (F). If F=1 is defined as the casewhen the sample was placed without any protection, then F shows howeffectively the debris shield protection works. Experimental results forthe 1 and 2 cm thick single channel setup are plotted in FIG. 2A(17).These results show a reduction factor of 100 per cm of shield length.These results may be compared with the reduction factor measured for amore realistic multi-channel debris shield shown in FIG. 2A(16). Thisprototype shield was fabricated from stainless steel by electrondischarge machining (EDM). The data show that under these conditions thereduction factor measured for the 1 cm long multi-channel shield wascomparable to the simple 1 cm single channel setup. This gives us ameasure of confidence in scaling this type of debris shield to thelength required for practical source operation.

Thermal Engineering

[0224] Water-cooled electrodes, the first step in development of athermal management solution for the DPF discharge region, have beendesigned and tested on Applicants fourth generation EUV light source.These electrodes have enabled study of the DPF operation atsignificantly higher steady-state repetition rates than previouslyachieved and generated calorimetric data that shows the dissipation ofthermal energy in each electrode.

[0225] The cathode has four separate cooling delivery and exhaust loops,one for each quadrant of the annular weldment. The flow through eachquadrant is arranged to be similar. It was designed to maximize the areacooled internally by the water and minimize the conduction path throughthe plasma heated wall and was fabricated from a high thermalconductivity copper alloy with good mechanical properties. At 400 kPathe total water flow through the cathode is 3.8 liters per minute. Thewater-cooled electrodes are shown diagrammatically in FIG. 2A(18). Theanode is cooled by flowing water through two concentric, annularchannels created in the body of its welded assembly. This allows thewater to get very close to the region of the part heated mostaggressively by the plasma. Water can be pumped through this electrodeat relatively high pressures giving high water flow rates andmaintaining a more favorable temperature gradient in the region ofhighest heat flux. In recent testing water has been pumped through theanode at 1100 kPa giving a flow rate of 11 liters per minute.

[0226] Testing of the water-cooled electrodes has been carried out up toseveral hundred Hertz in short bursts and at steady state repetitionrates up to 200 Hz. The results so far indicate that a reasonablecorrelation exists between measured electrical energy input and measuredheat load on the electrode cooling system when other as yet unmeasuredbut largely understood system heat losses are considered. The thermalenergy leaving the electrodes in the water is not divided evenly betweenthe anode and cathode. Typically the cathode removes more heat than theanode. The data suggest that the cathode removes a higher proportion ofthe heat as the repetition rate rises. This was expected since the anodetemperature rises more rapidly than that of the cathode with increasingrepetition rate and the corresponding reduction of thermal conductivityin the anode material is significant. The cathode also has a much largercooled area, a shorter heat conduction path and far higher thermalconductivity than the anode. The fraction of heat removed by eachelectrode is shown in FIG. 2A(19).

[0227] A summary of the demonstrated source parameters is given in FIG.A(20). In the past year Applicants have built five new DPF sources aswell as implementing upgrades to our existing fourth generation systembringing the total number of operational systems at Cymer to six.Significant improvements were made in the conversion efficiencyprimarily by optimization of the gas recipe and gas injection geometry.The best achieved conversion efficiency into 2 π sr and 2% bandwidth was0.4% at 10.5 J and low repetition rate. Stable EUV output wasdemonstrated for 300 pulse bursts at 2 kHz using our proven resonantcharger technology. Experiments performed to date suggest that furtherimprovement is possible by continued optimization of the gas deliverysystem. Energy stability continues to be ˜10% (1 σ) and will requireimprovement. Out of band radiation is <0.5% for the improved CE source.

[0228] Characterization of debris collected on witness samples exposedto the pinch shows deposition primarily of anode material (W), and anodeinsulator material (Al, O). No evidence of cathode material is seen.Measurements of the debris reduction factor for single and multiplechannel debris shield show a reduction factor of 100× per cm of shieldlength. Extrapolating this result to a reduction factor of 108 suggeststhat a 4-5 cm shield will be required.

[0229] The measurements of heat extraction from the electrodes forcontinuous operation at 200 Hz show that approximately 60% of the poweris dissipated in the cathode with 40% going to the anode. This suggeststhat at 5000 Hz repetition rate and 10 J total input energy we wouldneed to extract approximately 20 kW from the anode electrode. At theseconditions using 0.4% CE we calculate a total in band radiated power of200 W into 2% BW and 2 π sr at the source. Appropriate reduction factorsmust be used for all downstream components that attenuate the sourceradiation.

Other Improvements Dual Purpose Collectors

[0230] Due to large reflection losses of EUV mirrors, minimization ofthe number of mirrors is very desirable for illumination systems for EUVlithography. Specially designed surfaces can have additional featuressuch as beam homogenization features. One such feature could be areflective diffuser added to a grazing incidence collector of the typedescribed above.

Use of Magnetic Field and Preionizers to Control Pinch

[0231] Applicants have demonstrated that magnetic fields can be used tocontrol the pinch size and position. In one embodiment a permanentmagnetic positioned above the pinch region reduces the pinch length.Magnets can also be positioned in the aode as shown in FIG. 28A.Magnetic fields can also be applied to help confine the pinch.Applicants have also demonstrated that the shape and position of thepinch can also be controlled by moderating the preionization signal frompreionizers 138 as shown in FIG. 2A(2).

Metal in Solution Target

[0232] Metals such as lithium and tin provide vapors which make goodactive gases to produce radiation in the 13.5 nm range. However, dealingwith metal vapors is difficult. A technique for providing targetmaterial at the pinch site is to form a liquid solution with the metaland inject the target in liquid form.

[0233] When a liquid solution containing the metal is inserted into thedischarge chamber, the metal does not have to be heated for delivery.The target delivery can be made in a so-called mass-limited way, i.e.,just the right amount of metal (particles) is delivered, no more massthan needed. This leaves no extra particles, which would otherwise justrepresent unwanted debris produced by the source. The target materialcan be delivered in a liquid jet from a nozzle, if a sufficiently highbacking pressure is applied. In this way, it can be delivered to thedischarge region and it can be avoided that the whole discharge chamberis filled with target material. Since colloidal particles in suspensionor liquids or particles in liquids are used, the target density can bemuch higher than for metal vapor. By choosing the right concentration ofmetal content of the liquids, an optimized mass-limited metal target canbe provided. It is also much simpler to just inject a liquid into thechamber rather than constructing a metal vapor delivery system, forinstance based on a heat pipe principle. Tin nitrate should be anefficient target for 13.5 nm to 14 nm EUV light generation.

[0234] An improvement in EUV output and preionization was observed whena pulsed magnetic field was applied by means of a coil mounted as shownin FIG. 28B below. The coil current pulse is shown in FIG. 29C. Thispulse produces a magnetic field between 200 and 500 G at the end of theanode. An improvement in preionization was seen as shown by the anodewaveform in FIG. 29. The corresponding change in C2 waveform is shown inFIG. 29. The application of the pulsed field resulted in a higherpreionization density in the anode cathode region as evidenced by thedrop in anode voltage shown in FIG. 29A. The EUV output increased withthe pulsed field. The in band EUV waveshape is shown in FIG. 34C with Bon and off. The overall dependence of the EUV output on input energywith pulsed field applied is shown in the upper curve of FIG. 34C. Thecurves below this are with no pulsed B field. FIG. 2A(9) showsimprovements in efficiency resulting from electrode geometry improvementdiscussed herein including gas pumping and preionization changes andplasma dynamics using magnetic effects.

[0235] Metal targets can be delivered by means of liquids, fluids,solutions or suspensions. The compound has to be liquid at the given(backing) pressure at temperatures around room temperature, say, from˜10° C. to ˜50° C. This technique applies to any pinched (=magneticallyself-compressed) discharge which can produce EUV or X-ray radiation,like a dense plasma focus (DPF), a Z-pinch, an HCT-pinch (=hollowcathode triggered pinch) or a capillary discharge. The liquid can bedelivered through the former gas injection port of the discharge device,see FIG. 18A for example for the case, when the discharge device is aDPF. In another embodiment see FIG. 23, the liquid can be at highpressure or can be backed up by very high-pressure (ca. 80 atm) heliumgas and be delivered to the discharge region via a jet nozzle with verysmall opening (ca. 50 μm to ca. 10 μm). In this way, themetal-containing liquid is confined to a narrow liquid jet. The jetcrosses the pinch region of the discharge. Additional gas may beinserted to promote the development of an efficient pinch discharge. Theliquid and evaporated gas can be pumped away by a nearby dump port witha vacuum pump. The nozzle expansion through the nozzle or through theinner electrode may also alternatively be operated such as to form aseries of liquid drops or as a (more diffuse) liquid spray expansion.The liquids provide an easy means of delivering metals of optimalconcentration, diluted in solution, to the discharge region. Heating ofthe metal to provide a metal vapor can be avoided.

[0236] The preferred metals are the ones that provide efficient EUVgeneration in the region of ca. 13 nm to ca. 15 nm. They are: lithium,tin, indium, cadmium and silver. Lithium (Li2+) has a strong transitionat 13.5 nm. Tin (Sn), indium (In), cadmium (Cd) and silver (Ag) havestrong 4d-4f transition arrays from several ion species overlapping inthe 13 to 15 nm wavelength region. (As one goes from 13 nm to 15 nm, thepeak reflectivity of the multi-layer mirrors for EUV lithographydecreases, but their bandwidth increases at the same time. Therefore,the integral reflected intensity can still be large, and wavelengthsabove 14 nm are still of interest here.) The preferred solutions arealcohols like iso-propanol, methanol, ethonol, etc., and also water orglycol.

[0237] The preferred chemical compounds are lithium fluoride, lithiumchloride, lithium bromide -salts, dissolved in water, for instance. ForSn, In, Cd and Ag preferred solutions are likewise chlorine solutions,bromine solutions and fluorine compounds. In addition, metal sulfatesand nitrates.

[0238] Tin nitrate (Sn(NO3)4) is one of the most interesting compounds.Likewise, indium nitrate (In(NO3)3), cadmium nitrate (Cd(NO3)2), andsliver nitrate (Ag(NO3)). Nano- and micro-particles in solution orsuspension may also be used. It may also be considered to insert suchnano- and micro-particles by turbulence into a gaseous stream of heliumand not use a liquid at all for delivery.

Additional EUV Light from Electron Impact

[0239] Applicants propose to supplement the in-band light produced byits plasma pinches with light results from energetic electron impact.

[0240] Bremsstrahlung (=soft x-ray radiation) generated from energeticelectron impact on solids with suitable absorption edges generates EUVradiation in addition to the EUV radiation produced in the gaseous pinchplasma. This is the idea in general. In the case of our DPF source, forinstance, it is known that when operated with positive polarity on thecentral electrode (=anode), an electron beam (with electron energies ofseveral keV is produced which impinges onto the inner front side of thecenter electrode. For 13.5 nm radiation, Si (silicon) is the suitablematerial to be placed here. The silicon L-absorption edge occurs at 13.5nm. Therefore, the energetic electrons will produce 13.5 nm radiation.This is completely in addition to the main 13.5 nm-radiation produced bythe xenon ions in the pinch plasma. Therefore, more EUV radiation willbe generated, if the central inner potion of the anode (in general, anyplace where the electron beam impacts) is made out of silicon. Anelectron kinetic energy of 10 keV is just about right for optimalefficiency. For instance, put silicon inside of the tungsten anode.Without silicon oat the place f impact (=present mode of operation),there is no match of the absorption edge (e.g., tungsten), consequentlyno additional radiation is produced at 13.5 nm. Silicon is of mostimportance here, but the principle applies also to other materials atother wavelengths. (For example: Beryllium insert to produce 11.5 nmradiation at the Be K edge). A sketch showing this technique is providedin FIG. 24.

Metal Vapor Produced by Sputtering

[0241] In preferred embodiments the active gas (lithium or tin vapor)and pre-ionization is provided in a single system. In this case themetal target is sputtered with an electric discharge which produces themetal vapor and also produces any ionization needed to promote the maindischarge. The source for the sputter power preferably is a signalgenerator, a 100 Watt linear RF amplifier and a 2000 Watt commandamplifier. The solid lithium or tin target is preferably located in ahollow in the central electrode and the sputter discharge are directedto that target.

[0242] For example, Applicants fourth generation EUV sources produceabout 5 Watts of in band EUV energy at the interim focus 11 in FIG. 19.Applicants expect future design using existing technology to boost this5 Watts to about 45.4 Watts. However, some designers of EUV lithographymediums have expressed a desire for power levels of more than 100 Watts.Applicants propose to accomplish this by combining two EUV sources usingthe technology described herein into one EUV system.

Wavelength Ranges

[0243] The various embodiments discussed herein have been discussedparticularly in terms of light sources producing ultraviolet in thespectral range of between 12 and 14 nm. This is because mirror suppliershave reported substantial success in the development of multi-layer nearnormal mirrors for UV light within these wavelengths ranges. Typicallythese mirrors have maximum reflectivities of about 0.6 to 0.7 in the 12to 14 nm range and the mirrors typically have a FWHM bandwidth of about0.6 nm depending on the specific mirror design. So the typical mirroronly covers a portion of the spectral range between 12 nm and 14 nm.

[0244] For this reason it is very important to carefully match thespectral output of the source to the spectral range of the reflectivityof the mirrors which will be used to direct the beam, such as themirrors in lithography scanner machine.

[0245] The reader should also understand that the teachings of thisspecification will apply to a much broader spectral range than the 12 nmto 14 nm range where most of the current extreme UV attention isfocused. For example, good mirrors can be produced for the 11 nm rangeand there may be advantageous for using these pinch devices atwavelengths above the 14 nm range up to about 50 nm. In the future itmay be possible to practice projection lithography down to about 5 nm.Also, by going to x-ray proximity lithography, it should be possible touse the techniques described herein for light sources down to about 0.5nm.

[0246] For projection lithography an active material would need to bechosen which would have at least one good emission line within thereflectivity range of the mirrors used for the projection good lines areavailable throughout extreme UV spectrum. Good lines are also availablein ranges which could apply down to 0.5 nm for the proximitylithography. Therefore, Applicants believe many or most of the conceptsand ideas expressed herein would be useful throughout the spectral rangefrom about 0.5 nm to about 50 nm.

[0247] It is understood that the above described embodiments areillustrative of only a few of the many possible specific embodimentswhich can represent applications of the principals of the presentinvention. For example, instead of recirculating the working gas it maybe preferable to merely trap the lithium and discharge the helium. Useof other electrode - coating combinations other than tungsten and silverare also possible. For example copper or platinum electrodes andcoatings would be workable. Other techniques for generating the plasmapinch can be substituted for the specific embodiment described. Some ofthese other techniques are described in the patents referenced in thebackground section of this specification, and those descriptions are allincorporated by reference herein. Many methods of generating highfrequency high voltage electrical pulses are available and can beutilized. An alternative would be to keep the lightpipe at roomtemperature and thus freeze out both the lithium and the tungsten as itattempts to travel down the length of the lightpipe. This freeze-outconcept would further reduce the amount of debris which reached theoptical components used in the lithography tool since the atoms would bepermanently attached to the lightpipe walls upon impact. Deposition ofelectrode material onto the lithography tool optics can be prevented bydesigning the collector optic to re-image the radiation spot through asmall orifice in the primary discharge chamber and use a differentialpumping arrangement. Helium or argon can be supplied from the secondchamber through the orifice into the first chamber. This scheme has beenshown to be effective in preventing material deposition on the outputwindows of copper vapor lasers. Lithium hydride may be used in the placeof lithium. The unit may also be operated as a static-fill systemwithout the working gas flowing through the electrodes. Of course, avery wide range of repetition rates are possible from single pulses toabout 5 pulses per second to several hundred or thousands of pulses persecond. If desired, the adjustment mechanism for adjusting the positionof the solid lithium could be modified so that the position of the tipof the central electrode is also adjustable to account for erosion ofthe tip.

[0248] Many other electrode arrangements are possible other than theones described above. For example, the outside electrode could be coneshaped rather than cylindrical as shown with the larger diameter towardthe pinch. Also, performance in some embodiments could be improved byallowing the inside electrode to protrude beyond the end of the outsideelectrode. This could be done with spark plugs or other preionizers wellknown in the art. Another preferred alternative is to utilize for theouter electrode an array of rods arranged to form a generallycylindrical or conical shape. This approach helps maintain a symmetricalpinch centered along the electrode axis because of the resultinginductive ballasting.

[0249] Accordingly, the reader is requested to determine the scope ofthe invention by the appended claims and their legal equivalents, andnot by the examples which have been given.

What is claimed is:
 1. A production line compatible, high repetitionrate, high average power pulsed high energy photon source comprising: A.a pulse power system comprising a pulse transformer for producingelectrical pulses with duration in the range of 10 ns to 200 ns, B. avacuum chamber, C. an active material contained in said vacuum chambersaid active material comprising an atomic species characterized by anemission line within a desired extreme ultraviolet wavelength range, D.a hot plasma production means for producing a hot plasma at a hot plasmaspot is said vacuum vessel so as to produce at least 5 Watts, averagedover at least extreme ultraviolet radiation at wavelengths within saiddesired extreme ultraviolet wavelength range, E. a radiation collectionand focusing means for collecting a portion of said ultravioletradiation and focusing said radiation at an location distant from saidhot plasma spot.
 2. A source as in claim 1 wherein said hot plasmaproduction means is a dense plasma focus device.
 3. A source as in claim1 wherein said hot plasma production means is a convention z-pinchdevice.
 4. A source as in claim 1 wherein said hot plasma productionmeans is a hollow cathode z-pinch.
 5. A source as in claim 1 whereinsaid hot plasma production means is a capillary discharge device.
 6. Asource as in claim 1 wherein said hot plasma production means comprisesan excimer laser providing a high repetition rate short pulse laser beamfor generating said plasma in said vacuum vessel.
 7. A source as inclaim 1 wherein said hot plasma production means comprises a plasmapinch device and an excimer laser producing pulsed ultraviolet laserbeams directed at a plasma produced in part by said plasma pinch device.8. A source as in claim 1 wherein said radiation collector comprises aparabolic collector.
 9. A source as in claim 1 wherein said radiationcollector comprises an ellipsoidal collector.
 10. A source as in claim 1wherein said radiation collector comprises a tandem ellipsoidal mirrorsystem.
 11. A source as in claim 1 wherein said radiation collectorcomprises a hybrid collector comprising at least one ellipsoidalreflector unit and at least one hyperbolic reflector unit.
 12. A sourceas in claim 11 wherein said hybrid collector comprises at least twoellipsoidal reflector units and at least two hyperbolic collector units.13. A source as in claim 12 wherein said hybrid collector also comprisesa multi-layer mirror unit.
 14. A source as in claim 13 wherein saidmulti-layer mirror unit is at least partially parabolic.
 15. A source asin claim 1 and also comprising a debris shield having narrow passagesaligned with said hot plasma spot for passage of EUV light andrestricting passage of debris.
 16. A source as in claim 15 wherein saiddebris shield is comprised of hardened material surrounding passage waysleft by removal of skinny pyramid shaped forms.
 17. A source as in claim15 wherein said debris shield is comprised of welded hollow cones iscomprised of metal foil.
 18. A source as in claim 15 wherein said debrisshield is comprised of a plurality of thin laminated sleek slashed tocreate said passageways.
 19. The source as in claim 15 and alsocomprising a magnet for producing a magnetic field directedperpendicular to an axis of EUV beams for forcing charged particles intoa curved trajectory
 20. The source as in claim 19 wherein said magnet isa permanent magnet.
 21. The source as in claim 19 wherein said magnet isan electromagnet.
 22. The source as in claim 15 wherein said debrisshield is a honeycomb debris shield.
 23. The source as in claim 22wherein said honeycomb debris shield comprises hardened plasticizedpowder batch material.
 24. The source as in claim 23 wherein said powderbatch material is hardened by sintering.
 25. The source as in claim 1wherein said active material is chosen from a group consisting of xenon,tin, lithium, indium, cadium and silver.
 26. The source as in claim 1wherein said vacuum contains, in addition to said active material, abuffer gas.
 27. The source as in claim 1 wherein said active material isinjected into said vacuum chamber through an electrode.
 28. The sourceas in claim 15 and further comprising a gas control system to creat agas flow in said vacuum vessel through at least a portion of said debrisshield in a direction opposite a direction of EUV light through saiddebris shield.
 29. The source as in claim 28 wherein gas flows throughsaid debris shield in two directions.
 30. The source as in claim 2wherein said dense plasma focus device comprises coaxial electrodes. 31.The source as in claim 30 and further comprising a gas injection meansfor injecting active gas from a nozzle positioned on an opposite side ofsaid hot plasma spot from said electrodes.
 32. The source as in claim 1wherein said active material is introduced into said vacuum chamber as acompound.
 33. The source as in claim 32 wherein the compound is chosenfrom a group consisting of LiO₂, LiH, LiOH, LiCl, Li₂Co₃, LiF, Ch₃ andsolutions of any materials in this group.
 34. The source as in claim 1and further comprising a laser for vaporizing said active material. 35.The source as in claim 1 and further comprising an RF source forsputtering active material into a location within or near said hotplasma spot.
 36. The source as in claim 1 and further comprising apreionization means.
 37. The source as in claim 1 wherein saidpreionization means comprises spark plug type pins.
 38. The source as inclaim 36 wherein said preionization means comprises an RF source. 39.The source as in claim 1 wherein said active material is preionizedprior to injection into said vacuum vessel.
 40. The source as in claim39 wherein said preionization means comprises a radiation means fordirecting radiation to a nozzle to preionize active material prior toits leaving said nozzle to enter said vacuum vessel.
 41. The source asin claim 26 wherein said buffer gas is chosen from a group consisting ofhelium and neon.
 42. The source as in claim 26 wherein said buffer gascomprises hydrogen.
 43. The source as in claim 2 and further comprisinga capacitor means chosen to produce peak capacitor current during aplasma pinch event.
 44. The source as in claim 2 wherein said denseplasma focus device comprise coaxial electrodes defining a centralelectrode.
 45. The source as in claim 44 wherein said central electrodeis an anode.
 46. The source as in claim 45 wherein a portion of saidanode is hollow and said anode defines a hollow tip dimension at a tipof said anode and said hollow portion below said tip is larger than saidhollow tip dimension.
 47. The source as in claim 1 wherein said activematerial is lithium contained in porous tungsten.
 48. The source as inclaim 47 and further comprising an RF means driving lithium atoms out ofsaid porous tungsten.
 49. The source as in claim 44 wherein said centralelectrode is water cooled.
 50. The source as in claim 44 and furthercomprising a heat pipe for cooling said central electrode.
 51. Thesource as in claim 44 wherein said electrodes are designed for radialrun down.
 52. The source as in claim 1 wherein said source is positionedto provide EUV light to a lithography machine.
 53. The source as inclaim 52 wherein a portion of said source is integrated into saidlithography machine.
 54. The source as in claim 44 and furthercomprising a sacrifice region between said electrode to encourage postpinch discharge in a region away from a tip of said anode.
 55. A sourceas in claim 44 and further comprising a sputter source for producingsputter material to replace material eroded from at least one of saidelectrodes.
 56. A source as in claim 56 wherein said sputter source alsofunctions to provide preionization.
 57. The source as in claim 44wherein said central electrode is an anode defining outside walls andfurther comprising insulator material completely covering anode wallsfacing said cathode.
 58. The source as in claim 57 wherein said anodealso defines inner walls and comprising insulator material covering atleast a portion of said inner walls.
 59. The source as in claim 44wherein said electrodes are comprised at least in part of pyrolyticgraphite.
 60. The source as in claim 1 and further comprising a shutterwith a seal located between said debris shield and said radiationcollector to permit replacement of electrodes and the debris shieldwithout loss of vacuum around said radiation collector.
 61. A source asin claim 44 and further comprising an electrode set arranged as a modulewith said debris shield so that the electrode set and the shield can beeasily replaced as a unit.
 62. A source as in claim 1 wherein said meansfor producing a hot plasma is sufficient to produce at least 45.4 Wattsat said intermediate focus.
 63. A source as in claim 1 wherein saidmeans for producing a hot plasma is sufficient to produce at least 105.8Watts at said intermediate focus.
 64. A source as in claim 1 whereinsaid active material is chosen to produce EUV radiation within awavelength band of about 2% of 13.5 nm.
 65. A source as in claim 1wherein said pulse power system is operating at repetition rates of atleast 6,000 pulses per second.
 66. A source as in claim 1 wherein saidpulse power system is operating at repetition rates of at least 10,000pulses per second.
 67. A source as in claim 1 wherein said radiationcollector is designed to produce homogenization of said EUV radiation.68. A source as in claim 2 and further comprising a magnetic means forapplying a magnetic field to control at least one pinch parameters. 69.A source as in claim 68 wherein said parameter is pinch length.
 70. Asource as in claim 68 wherein said parameter is pinch shape.
 71. Asource as in claim 68 wherein said parameter is pinch position.
 72. Asource as in claim 1 wherein said active material is delivered toregions of said lot plasma spot as a metal in fluid form.
 73. A sourceas in claim 1 wherein said fluid form is liquid.
 74. A source as inclaim 1 wherein said fluid form is a solution.
 75. A source as in claim1 wherein said fluid form is a suspension.
 76. A source as in claim 1wherein EUV light produced by electrons impact an electron material iscollected along with EUV light from said plasma hot spot.
 77. A sourceas in claim 1 wherein said active material is a metal vapor produced bysputtering.
 78. A source as in claim 1 wherein said active material ischosen to produce high energy radiation light in the range of 0.5 nm to50 nm.